\ 

LIBRARY 

UNIVERSITY  OF    j 
^  CALIFORNIA^/ 


EARTH 

SCIENCE 

LIBRAR 


EARTH 
SCIENCES 
LIBRARY 


PRACTICAL  EXERCISES 


IN 


ELEMENTARY  METEOROLOGY 


BY 


EGBERT 


WAED 

M 


INSTBUCTOB  IN  CLIMATOLOGY  IN  HABVABD  UNIVEBSITY 


BOSTON,  U.S.A. 

GINN'&   COMPANY,  PUBLISHERS 
Cbe  &t&etuettm 
1899 


COPYRIGHT,  1899,  BY 
ROBERT  DECOURCY  WARD 

ALL  RIGHTS  RESERVED 


EARTH 
SCIENCES 
LIBRARY 


PEEFACE. 


THE  advance  of  meteorology  as  a  school  study  has  been  much 
hampered  by  the  lack  of  a  published  outline  of  work  in  this  sub- 
ject which  may  be  undertaken  during  the  school  years.  There 
are  several  excellent  text-books  for  more  advanced  study,  but 
there  is  no  laboratory  manual  for  use  in  the  elementary  portions 
of  the  science.  In  many  secondary  schools  some  instruction  in 
meteorology  is  given,  and  the  keeping  of  meteorological  records 
by  the  scholars  is  every  year  becoming  more  general.  There 
is  yet,  however,  but  little  system  in  this  work,  and,  in  conse- 
quence, there  is  little  definite  result.  The  object  of  this  book 
is  to  supply  a  guide  in  the  elementary  observational  and  induc- 
tive studies  in  meteorology.  This  Manual  is  not  intended  to 
replace  the  text-books,  but  is  designed  to  prepare  the  way 
for  their  more  intelligent  use.  Simple  preliminary  exercises  in 
the  taking  of  meteorological  observations,  and  in  the  study  of 
the  daily  weather  maps,  as  herein  suggested,  will  lay  a  good 
foundation  on  which  later  studies,  in  connection  with  the  text- 
books, may  be  built  up.  Explanations  of  the  various  facts 
discovered  through  these  exercises  are  not  considered  to  lie 
within  the  scope  of  this  book.  They  may  be  found  in  any.  of 
the  newer  text-books. 

This  Manual  lays  little  claim  to  originality.  Its  essential 
features  are  based  on  the  recommendations  in  the  Report  on 

Geography  of  the  Committee  of  Ten.     A  scheme  of  laboratory 

-      iii 


991447 


IV  PREFACE. 

exercises,  substantially  the  same  as  that  proposed  in  this  Report, 
was,  for  some  fifteen  years,  the  basis  of  the  work  in  elementary 
meteorology  done  in  Harvard  College  under  the  direction  of 
Professor  William  M.  Davis.  The  plan  proposed  by  the  Com- 
mittee of  Ten  has  been  thoroughly  tested  by  the  writer  during 
the  past  five  years,  not  only  in  college  classes,  but  also  in 
University  Extension  work  among  school  teachers,  and  the  pres- 
ent book  embodies  such  modifications  of  that  scheme  and 
additions  to  it  as  have  been  suggested  by  experience.  Emphasis 
is  laid  throughout  this  Manual  on  the  larger  lessons  to  be 
learned  from  the  individual  exercises,  and  on  the  relations  of 
various  atmospheric  phenomena  to  human  life  and  activities. 
No  attempt  is  made  to  specify  in  exactly  what  school  years  this 
work  should  be  undertaken.  At  present,  and  until  meteorology 
attains  a  recognized  position  as  a  school  study,  teachers  must 
obviously  be  left  to  decide  this  matter  according  to  the  oppor- 
tunities offered  in  each  school.  The  general  outline  of  the 
work,  however,  as  herein  set  forth,  is  intended  to  cover  the 
grammar  and  the  high  school  years,  and  may  readily  be  adapted 
by  the  teacher  to  fit  the  circumstances  of  any  particular  case. 

This  book  contains  specific  instructions  to  the  student  as  to 
the  use  of  the  instruments ;  the  carrying  out  of  meteorological 
observations ;  the  investigation  of  special  simple  problems  by 
means  of  the  instruments ;  and  the  practical  use  of  the  daily 
weather  maps.  The  Notes  for  the  Teacher,  at  the  end  of  the 
book,  are  explanatory,  and  contain  suggestions  which  may  be 
useful  in  directing  the  laboratory  work  of  the  class. 

It  has  been  the  privilege  of  the  author  during  the  past  ten 
years  to  study  the  science  of  meteorology,  and  the  methods  of 


PREFACE.  V 

teaching  that  science,  under  the  constant  direction  of  Professor 
William  Morris  Davis,  of  Harvard  University.  To  Professor 
Davis  the  author  is  further  indebted  for  many  valuable  sugges- 
tions in  connection  with  the  arrangement  and  treatment  of  the 
subject-matter  of  this  book.  Thanks  are  due  also  to  Mr. 
William  H.  Snyder,  of  Worcester  (Mass.)  Academy,  and  to 
Mr.  John  W.  Smith,  Local  Forecast  Official  of  the  United 
States  Weather  Bureau,  Boston,  Mass.,  for  valued  criticisms. 


ROBERT    DEC.    WARD. 


HARVARD  UNIVERSITY,  CAMBRIDGE,  MASS., 
September,  1899. 


CONTENTS. 


INTRODUCTION. 

PAGE 
THE  IMPORTANCE  OF  METEOROLOGY  :    ITS  RELATIONS  TO  MAN   .  xi 


PART   I.  —  NON-INSTRUMENTAL   OBSERVATIONS. 

CHAPTER  I.  —  OBSERVATIONS  OF  TEMPERATURE,   WIND  DIRECTION  AND 

VELOCITY,  STATE  OF  SKY,  AND  RAINFALL .         1 


PART   II.  —  INSTRUMENTAL   OBSERVATIONS. 

CHAPTER  II.  —  ELEMENTARY  INSTRUMENTAL  OBSERVATIONS 11 

CHAPTER  III.  —  ADVANCED  INSTRUMENTAL  OBSERVATIONS 26 

PART   III.  —  EXERCISES   IN   THE   CONSTRUCTION   OF    WEATHER 

MAPS. 

CHAPTER  IV.  —  THE  DAILY  WEATHER  MAP 47 

CHAPTER  V.  —  TEMPERATURE 51 

CHAPTER  VI.  —  WINDS 70 

CHAPTER  VII.  —  PRESSURE 76 

CHAPTER  VIII.  —  WEATHER      .     ^     .     .     v     . 85 

PART   IV. —THE   CORRELATIONS   OF   THE    WEATHER    ELEMENTS 
AND    WEATHER   FORECASTING. 

CHAPTER  IX.  —  CORRELATION  OF  THE  DIRECTION  OF  THE  WIND  AND  THE 

PRESSURE 91 

CHAPTER  X.  —  CORRELATION  OF  THE  VELOCITY  OF  THE  WIND  AND  THE 

PRESSURE «...       93 

CHAPTER  XI.  — FORM  AND  DIMENSIONS  OF  CYCLONES  AND  ANTICYCLONES       96 

CHAPTER  XII.  —  CORRELATION   OF   CYCLONES   AND   ANTICYCLONES   AND 

THEIR  WIND  CIRCULATION 98 

vii 


viii  CONTENTS. 

PAGE 

CHAPTER  XIII.  —  CORRELATION  OF  THE   DIRECTION  OF  THE  WIND  AND 

THE  TEMPERATURE 101 

CHAPTER  XIV.  —  CORRELATION  OF  CYCLONES  AND  ANTICYCLONES  AND 

THEIR  TEMPERATURES 104 

CHAPTER  XV.  —  CORRELATION  OF  THE  DIRECTION  OF  THE  WIND  AND 

THE  WEATHER 106 

CHAPTER  XVI.  —  CORRELATION  OF  CYCLONES  AND  ANTICYCLONES  AND 

THE  WEATHER 109 

CHAPTER  XVII. — PROGRESSION  OF  CYCLONES  AND  ANTICYCLONES  .  .  Ill 

CHAPTER  XVIII. — SEQUENCE  OF  LOCAL  WEATHER  CHANGES  .  .  .  .  113 

CHAPTER  XIX.  —  WEATHER  FORECASTING 114 

PART  V.  —  PROBLEMS   IN   OBSERVATIONAL   METEOROLOGY. 

CHAPTER  XX.  —  TEMPERATURE 125 

CHAPTER  XXI.  —  WINDS 130 

CHAPTER  XXII.  —  HUMIDITY,  DEW,  AND  FROST       .    • 132 

CHAPTER  XXIII. — CLOUDS  AND  UPPER  AIR  CURRENTS 136 

CHAPTER  XXIV.  —  PRECIPITATION 138 

CHAPTER  XXV.  —  PRESSURE 139 

CHAPTER  XXVI. — METEOROLOGICAL  TABLES 142 

APPENDIX   A. 

SUGGESTIONS  TO  TEACHERS 171 

APPENDIX   B. 

THE  EQUIPMENT  OF  A  METEOROLOGICAL  LABORATORY  ....  186 


INDEX 197 


ACKNOWLEDGMENT  OF   FIGURES. 


1,  7,  8,   9,   10,   16.     Meteorological    Instruments.     H.    J.    Green, 

1191  Bedford  Avenue,  Brooklyn,  K  Y. 

2,  4.     Instrument  Shelter  and  Eain  Gauge.     Instructions  for  Volun- 

tary Observers.     United  States  Weather  Bureau. 

5.      Mercurial  Barometer.     L.  E.  Knott  Apparatus  Co.,  14  Ashbur- 
ton  Place,  Boston,  Mass. 

12,   15,   53.     Thermograph  and  Barograph  Curves,  and  Cyclonic 
Composite.     Davis,  Elementary  Meteorology. 

17.     Nephoscope.     Annals  Harvard  College  Observatory,  Vol.  XX, 
Part  I. 

48.     North  Atlantic  Cyclone.     Pilot  Chart  of  the  North  Atlantic 
Ocean.     United  States  Hydrographic  Office. 

51.     Wind  Rose.      Quarterly  Journal  Royal  Meteorological  Society, 
Vol.  XXIV,  No.  108. 


-\ 


INTRODUCTION. 

THE  IMPOETANCE   OF  METEOROLOGY :   ITS  RELATIONS  TO   MAN. 


WE  live  in  the  laboratory  of  the  earth's  atmosphere.  The 
changes  from  hot  to  cold,  wet  to  dry,  clear  to  cloudy,  or  the 
reverse,  profoundly  affect  us.  We  make  and  unmake  our  daily 
plans ;  we  study  or  we  enjoy  vacations ;  we  vary  our  amusements 
and  our  clothing  according  to  these  changes.  The  weather 
forecasts  for  the  day  in  the  newspaper  are  read  even  before 
the  telegraphic  despatches  of  important  events.  Sailors  about 
to  put  to  sea  govern  themselves  according  to  the  storm  warn- 
ings of  our  Weather  Bureau.  Farmers  and  shippers  of  fruit, 
meat,  and  vegetables  anxiously  watch  the  bulletins  of  cold  or 
warm  waves,  and  guard  against  damage  by  frost  or  excessive 
heat.  Steam  and  electric  railways  prepare  their  snow-plows 
when  a  severe  snowstorm  is  predicted. 

Meteorology,  the  science  of  the  atmosphere,  is  thus  of  very 
great  interest  and  importance.  There  is  no  subject  a  knowl- 
edge of  which  does  more  to  make  our  daily  life  interesting. 
Since  we  live  in  the  midst  of  the  atmosphere  and  cannot  escape 
from  the  changes  that  take  place  in  it,  we  must,  consciously  or 
unconsciously,  become  observers  of  these  changes.  Examples 
of  the  varying  processes  at  work  in  the  atmosphere  are  always 
with  us.  There  is  no  end  to  the  number  and  the  variety  of  our 
illustrations  of  these  processes.  Man  is  so  profoundly  affected 
by  weather  changes  from  day  to  day  that  all  civilized  countries 
have  established  weather  services.  Observers  taking  regular 
weather  records  are  stationed  at  thousands  of  different  places 
in  all  parts  of  the  world,  and  the  observations  which  they  make 


xii  INTRODUCTION. 

are  used  by  meteorologists  in  preparing  daily  weather  maps  and 
forecasts,  and  in  studying  the  conditions  of  temperature,  winds, 
and  rainfall.  In  the  United  States  alone  there  are  about  3000 
of  these  observers. 

These  observations  are  not  made  on  land  only.  Hundreds 
of  ship  captains  on  all  the  oceans  of  the  world  are  making  their 
regular  daily  meteorological  records,  which  at  the  end  of  the 
voyage  are  sent  to  some  central  office,1  where  they  are  studied 
and  employed  in  the  preparation  of  Pilot  Charts  for  the  use  of 
mariners.  By  means  of  these  ocean  meteorological  observations, 
which  were  first  systematized  and  carried  out  on  a  large  scale 
under  the  direction  of  Lieutenant  Matthew  Fontaine  Maury 
(born,  1806;  died,  1873),  of  the  United  States  Navy,  it  has 
become  possible  to  lay  out  the  most  favorable  sailing  routes  for 
vessels  engaged  in  commerce  in  all  parts  of  the  world. 

So  important  is  a  knowledge  of  the  conditions  of  the  winds 
and  the  weather,  that  scientific  expeditions  into  unexplored  or 
little-known  regions  give  much  of  their  time  to  meteorological 
observations.  On  the  famous  Lady  Franklin  Bay  Expedition 
(1881-1884)  of  Lieutenant  (now  General)  A.  W.  Greely,  of  the 
United  States  Army,  meteorological  observations  were  kept  up 
by  the  few  feeble  survivors,  after  death  by  disease  and  starvation 
had  almost  wiped  out  the  party  altogether,  and  when  those  who 
were  left  had  but  a  few  hours  to  live  unless  rescue  came  at 
once.  On  Nansen's  expedition  to  the  "  Farthest  North,"  on 
Peary's  trips  to  Greenland,  and  on  every  recent  voyage  to 
the  Arctic  or  the  Antarctic,  meteorological  instruments  have 
formed  an  important  part  of  the  equipment. 

Not  content  with  obtaining  records  from  the  air  near 
the  earth's  surface,  meteorologists  have  sent  up  their  instru- 
ments by  means  of  small,  un-manned  balloons  to  heights  of  10 
miles ;  and  the  use  of  kites  for  carrying  up  such  instruments 

1  In  the  United  States,  marine  meteorological  observations  are  forwarded  to 
the  United  States  Hydrographic  Office,  Navy  Department,  Washington. 

jov  ft   Ot'.Wfcr.  Si  V\r.  ft  , 


INTRODUCTION.  Xlll 

has  been  so  successful  that,  at  Blue  Hill  Observatory,  near 
Boston,  Mass.,  records  have  been  obtained  from  a  height  of 
over  2  miles.  Observatories  have  also  been  established  on 
mountain  summits,  where  meteorological  observations  have 
been  made  with  more  or  less  regularity.  Such  observatories 
are  those  on  Pike's  Peak,  Colorado  (14,134  feet),  Mont  Blanc, 
Switzerland  (15,780  feet),  and  on  El  Misti,  in  southern  Peru. 
The  latter,  19,200  feet  above  sea  level,  is  the  highest  meteoro- 
logical station  in  the  world. 

The  study  of  the  meteorological  conditions  prevailing  over 
the  earth  has  thus  become  of  world-wide  importance.  In  the 
following  exercises  we  shall  carry  out,  in  a  small  way,  inves- 
tigations similar  to  those  which  have  occupied  and  are  now 
occupying  the  attention  of  meteorologists  all  over  the  world. 


PRACTICAL    EXERCISES    IN    ELEMENTARY 
METEOROLOGY. 


PART  I.  —  NON-INSTRUMENTAL  OBSERVATIONS. 


CHAPTER   I.  V/A,:.,P/  V2 

OBSERVATIONS   OF   TEMPERATURE;    WIND   DIRECTION  AND 
VELOCITY;    STATE  OF   SKY,   AND  RAINFALL. 

BEFOKE  beginning  observations  with  the  ordinary  instruments, 
accustom  yourself  to  making  and  recording  observations  of  a 
general  character,  such  as  may  be  carried  out  without  the  use 
of  any  instruments  whatever.  Such  records  include :  Tem- 
perature ;  Wind  Direction  and  Velocity;  State  of  the  Sky^  arid 
Rainfall. 

Temperature.  —  In  keeping  a  record l  of  temperature  without 
the  use  of  a  thermometer,  excellent  practice  is  given  in  observa- 
tions of  the  temperature  actually  felt  by  the  human  body.  Our 
bodies  are  not  thermometers.  They  do  not  indicate,  by  our 
sensations  of  heat  or  cold,  just  what  is  the  temperature  of 
the  surrounding  air,  but  they  try  to  adjust  themselves  to  the 
conditions  in  which  they  are.  This  adjustment  depends  on 
many  things  beside  the  temperature  of  the  air ;  e.g.,  the  moisture 
or  humidity  of  the  air ;  the  movement  of  the  air ;  the  tempera- 
ture and  the  nearness  of  surrounding  objects.  In  summer,  a 
day  on  which  the  temperature  reaches  80°  or  85°  often  seems 
much  hotter  than  another  day  on  which  the  temperature  rises 

1  Each  scholar  will  need  a  blank  book  in  which  to  preserve  the  observations. 

1 


2  NON-INSTRUMENTAL   OBSERVATIONS. 

to  95°.  In  winter,  temperatures  registered  by  the  thermometer 
as  10°  or  15°  above  zero  often  feel  a  great  deal  colder  than 
temperatures  of  -  5°  or  -  10°.  In  recording  your  observations 
on  temperature,  the  record  book  may  be  divided  into  columns 
as  follows :  — 

SAMPLE  EECORD  or  TEMPERATURE. 


DATE. 

HOUR. 

TEMPER- 
ATURE. 

REMARKS. 

Jan.  16  , 

,  9  A.M. 

Chilly 

i'  '    I  V'J  *' 

12  M. 

Warmer 

i        n 

4CP.M. 

u 

Growing  slowly  warmer  all  day. 

•^  ,a-7; 

'.a  A.M. 

Warm 

About  the  same  as  Jan.  16,  4  P.M. 

a        u 

11  A.M. 

Cooler 

Began  to  grow  cooler  about  10  A.M. 

a        a 

3P.M. 

Colder 

Steadily  becoming  colder. 

The  following  are  some  of  the  questions  you  should  ask  your- 
self in  carrying  out  this  work.  It  is  not  expected  that  you  will 
be  able  to  answer  all  these  questions  at  once,  but  that  you  will 
keep  them  in  mind  during  your  studies,  and  try  to  discover  the 
answers,  as  a  result  of  your  own  observations. 

How  does  it  feel  to  you  out  of  doors  to-day?  Is  it  hot, 
warm,  cool,  or  cold?  What  is  the  difference  between  your 
feelings  yesterday  and  to-day?  Between  day  before  yesterday 
and  to-day?  Have  you  noticed  any  regular  change  in  your 
feelings  as  to  warmth  and  cold  during  three  or  four  successive 
days  ?  During  the  past  week  or  two  ?  During  the  past  month  ? 
Is  there  any  difference  between  the  temperature  of  morning, 
noon,  afternoon,  and  evening?  Is  there  any  regular  variation 
in  temperature  during  the  day?  Have  there  been  any  sudden 
changes  in  temperature  during  the  last  few  days  ?  Have  these 
sudden  changes  brought  warmer  or  cooler  weather?  Has  the 
warmer  or  cooler  weather  continued  for  a  day  or  so,  or  has 
another  change  quickly  followed  the  first?  Have  the  sudden 
changes,  if  you  have  noted  any,  come  at  any  regular  times  (as 


WIND    DIRECTION    AND    VELOCITY.  8 

morning,  afternoon,  evening)  or  at  irregular  intervals?  Does 
there  seem  to  you  to  be  any  definite  system,  of  any  kind,  in  our 
changes  of  temperature  ?  In  what  ways  are  people  in  general 
affected  by  hot  weather?  By  cold  weather?  What  difference 
does  a  very  hot  or  a  very  cold  day  make  in  your  own  case  ? 

Wind  Direction  and  Velocity. — Wind  is  an  important  meteor- 
ological element  because  it  has  many  close  relations  to  human 
life.  It  affects  very  markedly  our  bodily  sensations  of  heat  or 
cold.  A  cold,  calm  day  is  pleasanter  than  a  cold,  windy  day. 
On  the  other  hand,  a  hot,  calm  day  is  usually  much  more 
uncomfortable  than  a  hot,  windy  day.  High  winds  cause 
wrecks  along  seacoasts  and  damage  houses,  crops,  and  fruit 
trees.  Sea  breezes  bring  in  fresh,  cool,  pure  air  from  the  ocean 
on  hot  summer  days.  In  the  tropics  the  sea  breeze  is  so  important 
in  preserving  the  health  of  Europeans  in  many  places  that  it  is 
known  as  "  the  doctor."  The  movement  of  wind  through  large 
cities  carries  off  the  foul  air  which  has  collected  in  the  narrow 
streets  and  alleys,  and  is  thus  a  great  purifying  agent. 

Record  the  direction  of  the  wind  according  to  the  four  cardinal 
points  of  the  compass  (N.,  E.,  S.,  and  W.)  and  the  four  inter- 
mediate points  (NE.,  SE.,  SW.,  and  NW.).  The  direction  of 
the  wind  is  the  point  from  which  the  wind  blows.  You  can 
determine  the  points  of  the  compass  roughly  by  noting  where 
the  sun  rises  and  where  it  sets. 

Note  the  velocity  of  the  wind  according  to  the  following  scale, 
proposed  by  Professor  H.  A.  Hazen  of  the  United  States  Weather 
Bureau. 

0  CALM. 

1  LIGHT  ;  just  moving  the  leaves  of  trees. 

2  MODERATE;  moving  branches. 

3  BRISK  ;  swaying  branches  ;  blowing  up  dust. 

4  HIGH  ;  blowing  up  twigs  from  the  ground,  swaying  whole  trees. 

5  GALE  ;  breaking  small  branches,  loosening  bricks  on  chimneys. 
•6  HURRICANE  or  TORNADO  ;  destroying  everything  in  its  path. 


4  NON-INSTRUMENTAL    OBSERVATIONS. 

The  record  book  will  need  two  additional  columns  when  wind 
observations  are  begun,  as  follows  :  - 

SAMPLE  RECORD  OF  TEMPERATURE  AND  WIND. 


DATE. 

Ho  UK. 

TEMPER- 
ATURE. 

WIND 
DIRECTION. 

WIND 
VELOCITY. 

REMARKS. 

Oct.  3 

7.30  A.M. 

Cool 

NE. 

Moderate 

Temperature  falling 

since  last  evening. 

« 

Wind    velocity  in- 

creasing. 

u     u 

11  A.M. 

|( 

« 

Brisk 

Temperature  the 

same.    Wind  veloc- 

ity still  increasing. 

U        It 

3  P.M. 

Li. 

High 

Wind  velocity   still 

increasing. 

What  is  the  direction  of  the  wind  to-day?  What  is  its 
velocity  ?  Has  its  direction  or  velocity  changed  since  yester- 
day? If  so,  was  the  change  sudden  or  gradual?  Have  you 
noticed  any  calms  ?  What  was  the  direction  of  the  wind  before 
the  calm  ?  What  after  the  calm  ?  Does  there  seem  to  be  more 
wind  from  one  compass  point  than  from  another  ?  Is  there  any 
relation  between  the  direction  of  the  wind  and  its  velocity  ?  i.e., 
is  the  NW.  wind,  for  instance,  usually  a  brisk  or  a  high  wind, 
or,  is  the  SE.  or  S.  wind  usually  moderate?  Does  the  wind 
usually  change  its  direction  gradually,  as  from  SE.  to  S.,  then 
to  SW.,  then  to  W.,  etc.,  or  does  it  jump  all  at  once,  as  from 
SE.  to  W.  ?  Is  there  any  relation  between  the  velocity  of 
the  wind  and  the  hour  of  the  day,  i.e.,  does  the  wind  seem 
stronger  or  weaker  at  noon  than  in  the  morning  or  at 
night?  Is  it  a  common  occurrence  to  have  a  wind  from 
the  same  direction  for  several  successive  days,  or  are  we 
apt  to  have  different  winds  almost  every  day  ?  Do  you 
notice  any  systematic  changes  in  wind  direction  which  are 


STATE    OF    THE    SKY.  5 

often  repeated?  What  are  these  changes?  Can  you  make 
a  simple  rule  for  them?  In  what  ways  does  the  wind 
affect  us? 

State  of  the  Sky.  —  By  the  state  of  the  sky  is  meant  the  condi- 
tion of  the  sky  as  to  its  cloudiness.  Clouds  add  much  to  the 
beauty  and  variety  of  nature.  They  are  often  gorgeously 
colored  at  sunset.  By  their  changes  in  form,  color,  and  amount 
from  day  to  day  they  relieve  what  might  otherwise  be  a  weari- 
some succession  of  the  same  weather  types.  Prevailingly  over- 
cast skies  have  a  depressing  effect.  Prevailingly  clear  skies 
become  monotonous.  A  proper  amount  of  bright  sunshine  is 
essential  for  the  ripening  of  crops,  but  too  much  sunshine  may 
parch  soil  and  vegetation,  and  become  injurious.  Clouds  bring 
rain  ;  hence  a  sufficient  amount  of  cloudiness  is  just  as  neces- 
sary as  a  sufficient  amount  of  sunshine.  The  drift  of  clouds 
shows  us  the  direction  of  movement  of  the  air  above  us,  and  is 
of  considerable  help  in  forecasting  the  weather.  Fog,  which  is 
a  very  low  cloud,  is  in  some  cases  so  common  as  to  be  a  mete- 
orological element  of  great  importance.  In  the  city  of  London, 
where  fogs  are  very  prevalent,  especially  in  winter,  the 
average  number  of  hours  of  bright  sunshine  in  December 
and  January  is  only  fifteen  in  each  month.  The  London  fogs 
are,  in  great  part,  due  to  the  presence  in  the  air  of  vast 
numbers  of  particles  of  soot  and  smoke  from  millions  of 
fires.  These  particles  increase  the  density  of  the  fog  and 
prolong  its  duration. 

The  amount  of  cloudiness  is  recorded  on  a  scale  of  tenths. 
A  clear  sky  is  one  that  is  less  than  T37  cloudy  ;  a  fair  sky  is 
from  -j3^-  to  -3-Q  cloudy ;  and  a  cloudy  sky  is  over  ^  cloudy.  In 
observing  the  state  of  the  sky,  note  such  points  as  the  times  of 
clouding  and  of  clearing ;  the  arrangement  of  the  clouds,  i.e., 
whether  they  are  few  and  scattered,  or  cover  the  sky  with  a 
uniform  layer  ;  the  common  forms  of  clouds  ;  the  changes  in 
the  amounts  of  cloudiness,  etc. 


6  NON-INSTRUMENTAL    OBSERVATIONS. 

Another  new  column  must  be  added  in  the  record  book  for 
the  cloudiness.     The  table  will  now  appear  thus  :  - 

SAMPLE  RECORD  OF  TEMPERATURE,  WIND,  AND  STATE  OK  THE  SKY. 


DATE. 

HOUR. 

TEMPER- 
ATURE. 

WIND. 

STATE 
OF  SKY. 

REMARKS. 

DIRECTION. 

VELOCITY. 

Dec.  18 

9  A.M. 

Very  cold 

NW. 

Brisk 

Clear 

Very  cold  all  night. 

Everything  frozen 

up. 

U          11 

5  P.M. 

a       n 

(i 

it 

K 

Same  conditions. 

"     19 

8.30  A.M. 

A  little 

« 

Moderate 

Fair 

Wind  less  violent. 

warmer 

Small  clouds  scat- 

tered over  the  sky. 

Is  the  sky  clear,  fair,  or  cloudy  to-day?  Is  there  more  or 
less  cloud  than  there  was  yesterday  ?  Than  day  before  yester- 
day ?  Is  to-day  a  day  of  increasing  or  of  decreasing  cloudiness  ? 
Is  the  sky  usually  perfectly  clear,  or  is  it  oftenest  somewhat 
clouded  over  ?  How  long  does  it  take  for  the  sky  to  become 
completely  covered  with  clouds  from-  the  time  when  it  first 
begins  to  become  cloudy  ?  When  there  are  a  few  clouds  in  the 
sky,  are  these  usually  scattered  all  over  the  sky,  or  are  they 
in  groups?  Have  you  noticed  any  particular  form  of  clouds 
which  seemed  familiar  to  you?  Do  clouds  seem  to  have 
certain  definite  shapes  and  appearances  which  are  to  be 
seen  often?  Do  you  discover  any  variation  of  cloudiness 
during  the  day,  i.e.,  is  it  apt  to  be  more  cloudy  in  the 
afternoon  than  in  the  morning  or  at  night?  Can  you 
make  a  list  describing  some  of  the  clouds-  that  you  see 
most  often?  Can  you  give  these  common  kinds  of  clouds 
some  names  of  your  own  that  shall  describe  them  briefly? 
In  what  ways  does  a  clear  sky,  with  bright  sunshine, 
affect  us? 


RAINFALL.  7 

Rainfall,  —  Under  the  general  term  rainfall,  meteorologists 
include,  besides  rain  itself,  snow,  hail,  sleet,  etc.  The  term 
precipitation  is  also  often  used.  Rainfall  stands  in  close  rela- 
tion to  human  life  and  occupations.  It  feeds  lakes  and  rivers, 
thus  furnishing  means  of  transportation,  power  for  running 
mills  and  factories,  and  water  supplies  for  cities.  Regions  of 
abundant  rainfall  are  usually  heavily  forested,  like  the  Amazon 
valley  in  South  America,  and  parts  of  Equatorial  Africa.  In 
civilized  countries  lumbering  is  apt  to  be  an  important  occu- 
pation in  districts  of  heavy  rainfall,  as  in  Oregon  and  Wash- 
ington in  our  own  country,  and  in  Southern  Chile  in  South 
America.  Where  there  is  a  moderate  rainfall,  and  other 
conditions  are  favorable,  there  agriculture  is  possible,  and 
farming  becomes  one  of  the  chief  occupations,  as  in  the 
Mississippi  and  Missouri  valleys  in  the  United  States,  and 
in  Western  Canada.  Districts  which  have  a  rainfall  too 
small  for  successful  agriculture,  but  are  not  by  any  means 
deserts,  are  often  excellent  grazing  lands,  as  in  the  case  of 
parts  of  Texas,  Nebraska,  and  Kansas  in  the  United  States, 
and  the  Argentine  Republic  in  South  America.  Where  there 
is  very  little  rainfall  deserts  are  found.  Cities  are  not  built 
in  deserts,  because  there  are  no  occupations  to  attract  large 
numbers  of  men.  The  inhabitants  of  the  desert  are  wander- 
ing tribes,  which  move  from  place  to  place  in  search  of  water 
and  food  for  themselves  and  their  animals.  Rain  and  snow 
cleanse  the  air,  washing  out  impurities  such  as  dust  and 
smoke.  Hence  they  are  important  agents  in  preserving 
health. 

Note  the  kind  of  precipitation  (rain,  snow,  hail,  sleet) ;  the 
amount  (heavy,  moderate,  light,  trace)  ;  and  the  time  of  the 
beginning  and  ending  of  the  storm  or  shower. 

The  record  book  must  now  be  further  subdivided  into 
columns,  to  make  room  for  the  rainfall  observations,  in  this 
manner  :  — 


8  NON-INSTKUMENTAL    OBSERVATIONS. 

SAMPLE  RECORD  OF  TEMPERATURE,  WIND,  STATE  OF  SKY,  AND  PRECIPITATION. 


WIND. 

PRECIPITATION. 

DATE. 

HOUR. 

TEMPER- 
ATURE. 

DIREC- 

VELOC- 

OF SKY. 

TIME 

OF 

KIND. 

AM'T. 

REMARKS. 

TION. 

ITY. 

GINN. 

Mar.  21 

8.30A.M. 

Mild 

S. 

Light 

Over- 

8A.M. 

Rain 

Light 

Raining. 

cast 

«      « 

12  M. 

M 

" 

« 

Over- 

" 

" 

" 

cast 

i<      i. 

4P.M. 

» 

« 

Moder- 

Over- 

Stopped raining 

ate 

cast 

about  3  P.M. 

"     22 

8A.M. 

Cool 

NW. 

Brisk 

Clear 

Cleared  off  dur- 

ing the  night. 

Does  most  of  our  rain  come  in  brief  showers,  or  in  storms 
lasting  a  day  or  two  ?  Do  we  have  about  the  same  amount  of 
rain  or  snow  every  week  and  every  month,  or  does  the 
amount  vary  a  good  deal  from  week  to  week  and  from  month 
to  month?  Do  you  notice  much  difference  in  the  character- 
istics of  successive  storms,  or  do  they  all  seem  pretty  much 
alike  ?  Are  thunderstorms  limited  to  any  particular  season  of 
the  year  ?  If  so,  to  what  season  ?  Have  you  discovered  any 
rule  as  to  the  time  of  day  when  rainstorms  or  snowstorms 
begin  ?  When  thunderstorms  begin  and  end  ?  Is  it  common 
or  uncommon  for  us  to  have  a  storm  lasting  three  or  four  days  ? 
How  long  does  a  thunderstorm  usually  last?  Do  we  have 
most  hail  in  winter  or  in  summer  ?  In  what  ways  does  a  rainy 
day  affect  people  ?  How  are  you  yourself  affected  ?  How  does 
a  heavy  snowstorm  affect  travel  and  transportation  ?  In  what 
ways  does  a  snowstorm  differ  from  a  rainstorm  as  to  the  charac- 
ter of  the  precipitation  and  its  effects  ? 

After  studying  the  temperature,  wind,  state  of  sky,  and  rainfall 
separately,  take  two  elements  together  and  see  what  relation  one 
has  to  the  other.  Try  to  answer  such  questions  as  these  : 

Temperature  and  Wind.  —  What  relations  can  you  discover 
between  the  direction  of  the  wind  and  the  temperature  ?  Which 


WIND    AND    STATE    OF    SKY.  9 

winds  are  the  coolest  ?  Which  the  warmest  ?  Does  a  hot,  calm 
day  seem  warmer  or  cooler  than  a  hot,  windy  day  ?  Does  a  cold, 
calm  day  seem  colder  or  warmer  than  a  cold,  windy  day  ?  Does 
the  velocity  of  the  wind  have  any  effect  on  your  feeling  of  cold 
or  of  warmth  ?  If  so,  what  effect  ? 

Wind  and  State  of  Sky.  —  Has  the  direction  of  the  wind  any- 
thing to  do  with  the  cloudiness  ?  Is  there  more  apt  to  be  con- 
siderable cloudiness  with  wind  from  one  direction  than  from 
another?  What  winds  are  usually  accompanied  by  the  largest 
amount  of  cloud  ?  What  winds  usually  blow  when  the  sky  is 
clear?  Is  the  relation  of  cloudiness  to  certain  wind  directions 
so  close  that,  if  you  know  the  wind  direction,  you  can  make  a 
prediction  as  to  the  probable  cloudiness  ?  Are  the  winds  with 
clouds  more  common  in  one  month  than  another?  In  one 
season  than  another  ?  If  so,  which  month  ?  which  season  ? 

Temperature  and  State  of  Sky.  —  Do  you  notice  any  relation 
between  the  temperature  and  the  state  of  the  sky?  In  winter 
are  our  coldest  days  usually  cloudy  or  clear  ?  In  summer  are 
our  hottest  days  cloudy  or  clear?  Are  the  winds  that  give. us 
the  most  cloudiness  warm  or  cold  winds  in  whiter  and  in  sum- 
mer ?  Is  a  cloudy  night  colder  or  warmer  than  a  clear  night  ? 
Is  a  cloudy  day  colder  or  warmer  than  a  clear  day  ? 

State  of  Sky  and  Precipitation.  —  How  is  rainfall  or  snowfall 
related  to  the  cloudiness  ?  Do  we  ever  have  rain  or  snow  when 
the  sky  is  not  completely  covered  with  clouds  ?  Does  the  sky 
usually  become  quickly  covered  with  clouds  before  a  rain? 
Does  a  sky  wholly  covered  with  clouds  always  give  us  rain  or 
snow  ?  Does  the  sky  clear  rapidly  or  slowly  after  a  rain  ?  Are 
any  particular  kinds  of  clouds  associated  with  rain  or  with 
snowstorms  ?  With  brief  showers  ?  With  thunderstorms  ? 

Wind  and  Precipitation.  — Are  any  particular  wind  directions 
more  likely  than  others  to  give  us  rain  or  snow  ?  Are  these  the 
same  winds  as  those  which  give  us  the  most  cloudiness  ?  What 
winds  are  they  ?  Has  the  velocity  of  the  wind  any  relation  to 


10  NON-INSTKUMENTAL    OBSERVATIONS. 

the  rain  or  snowstorm?  Does  the  wind  blow  harder  before, 
during,  or  after  the  rain  or  snow?  What  changes  of  wind 
direction  have  you  noted  before,  during,  and  after  any  storm  ? 
Have  you  noticed  these  same  changes  in  other  storms  ?  Are 
they  so  common  in  our  storms  that  you  can  make  a  rule  as  to 
these  changes  ? 

Temperature  and  Precipitation.  — Does  a  shower  or  a  rain- 
storm in  the  hotter  months  affect  the  temperature  of  the  air 
in  any  way  ?  How  ?  In  the  winter  does  the  temperature  show 
any  changes  before  a  snowstorm?  Is  it  usually  warmer  or 
colder  then  than  a  day  or  two  before  the  storm  and  the  day 
after  ?  Is  it  usually  uncomfortably  cold  during  a  snowstorm  ? 
Are  rainy  spells  in  the  spring  and  the  autumn  months  cooler  or 
warmer  than  clear  dry  weather? 


PART   II.  —  INSTRUMENTAL    OBSERVATIONS. 


CHAPTER   II. 

ELEMENTARY   INSTRUMENTAL   OBSERVATIONS. 

THE  non-instrumental  observations,  suggested  in  the  preced- 
ing chapter,  prepare  the  way  for  the  more  exact  records  of  the 
weather  elements  which  are  obtainable  only  by  the  use  of  instru- 
ments. The  non-instrumental  records  are  not  to  be  entirely 
given  up,  even  after  the  instrumental  work  and  the  weather- 
map  exercises  have  begun,  but  should  be  continued  throughout 
the  course.  Notes  on  the  forms  and  changes  of  clouds,  on  the 
times  of  beginning  and  ending,  and  on  the  character  of  the 
precipitation,  as  outlined  in  the  last  chapter,  and  other  observa- 
tions made  without  the  use  of  instruments,  are  an  essential  part 
of  even  the  most  advanced  meteorological  records. 

The  simpler  instruments  are  the  ordinary  thermometer,  the 
wind  vane,  the  rain  gauge,  and  the  mercurial  barometer  (in  a 
modified  form).  Observations  with  these  instruments,  although 
of  a  simple  character,  can  be  made  very  useful.  The  advance 
over  the  non-instrumental  observations,  which  latter  may  be 
termed  observations  of  sensation,  is  a  decided  one.  In  place 
of  the  vague  and  untrustworthy  statements  concerning  hot  and 
cold,  warm  and  cool  days,  we  now  have  actual  degrees  of  tem- 
perature to  serve  as  a  basis  for  comparison  of  day  with  day  or 
month  with  month.  The  measurements  of  rain  and  snowfall 
enable  us  to  study  the  amounts  brought  in  different  storms, 
the  average  precipitation  of  the  various  months,  etc.  The 
important  facts  of  change  of  pressure  now  become  known,  and 

11 


12 


INSTRUMENTAL    OBSERVATIONS. 


also  the  relation  of  these  changes  to  the  weather.  Just  as  we 
have,  in  the  earlier  work,  become  familiar  with  our  typical 
weather  changes  and  types,  so  we  shall  now  have  our  eyes 
opened  to  the  actual  values  of  the  temperatures  and  precipita- 
tion connected  with  these  changes. 

The  ordinary  thermometer  (Greek :   heat  measure),  the  most 

commonly  used  and  most  widely 
known  of  all  meteorological  in- 
struments, was  in  an  elementary 
form  known  to  Galileo,  and  was 
used  by  him  in  his  lectures  at  the 
University  of  Padua  during  the 
years  1592  to  1609.  Thermometers 
enable  us  to  measure  the  tempera- 
tures of  different  bodies  by  com- 
parison with  certain  universally 
accepted  standards  of  temperature. 
These  standards  are  the  freezing 
and  boiling  points  of  distilled  water. 
In  its  common  form  the  thermometer 
consists  of  a  glass  tube,  closed  at 
the  top,  and  expanding  at  its  lower 
end  into  a  hollow  spherical  or 
cylindrical  glass  bulb.  This  bulb  and  part  of  the  tube  are 
filled  with  mercury  or  alcohol.  As  the  temperature  rises, 
the  liquid  expands,  flows  out  of  the  bulb,  and  rises  in 
the  tube.  As  the  temperature  falls,  the  mercury  or  alcohol 
contracts,  and  therefore  stands  at  a  lower  level  in  the 
tube.  In  order  that  the  amount  of  this  rise  or  fall  may  be 
accurately  known,  some  definite  scale  for  measurement  must 
be  adopted.  The  scale  commonly  used  in  this  country  owes 
its  name  to  Fahrenheit  (born  in  Danzig  in  1686  ;  died  in  1736), 
who  was  the  first  to  settle  upon  the  use  of  mercury  as  the 
liquid  in  thermometers,  and  also  the  first  definitely  to  adopt 


FIG.  l. 


THE    THERMOMETER.  13 

two  fixed  points  in  graduating  the  scale.  The  division  of  this 
scale  into  180°  between  the  freezing  point  (32°)  and  the  boiling 
point  (212°)  seems  to  have  been  taken  from  the  graduation  of 
a  semicircle.  Fahrenheit  was  a  manufacturer  of  all  sorts  of 
physical  apparatus,  and  it  has  been  thought  probable  that 
he  had  some  special  facilities  for  dividing  his  thermometer 
tubes  into  180  parts.  Mercury  is  most  commonly  used  as  the 
liquid  in  thermometers,  because  it  readily  indicates  changes  of 
temperature,  and  because  over  most  of  the  world  the  winter 
cold  is  not  sufficient  to  freeze  it.  The  freezing  point  of 
mercury  is  about  40°  below  zero  (—  40°  F.).  Alcohol,  which 
has  a  much  lower  freezing  point,  is  therefore  used  in  ther- 
mometers which  are  to  be  employed  in  very  cold  regions. 
Alcohol  thermometers  must,  for  instance,  be  used  in  Northern 
Siberia,  where  the  mean  January  temperature  is  60°  below 
zero. 

The  temperature  which  meteorologists  desire  to  obtain  by 
the  ordinary  thermometer  is  the  temperature  of  the  free  air  in 
the  shade.  In  order  that  thermometers  may  indicate  this  tem- 
perature, they  must,  if  possible,  be  placed  in  an  open  space 
where  there  is  an  unobstructed  circulation  of  the  air,  and  they 
must  be  protected  from  the  direct  rays  of  the  sun.  They  are, 
therefore,  usually  exposed  inside  of  a  cubical  enclosure  of  wooden 
lattice  work,  in  an  open  space  away  from  buildings,  and  at  a 
height  of  4  to  10  feet  above  the  ground,  preferably  a  grass- 
covered  surface.  This  enclosure  is  called  the  shelter,  and  its 
object  is  to  screen  the  instrument  from  the  direct  and  reflected 
sunshine,  to  allow  free  circulation  of  air  around  the  bulb,  and 
to  keep  the  thermometer  dry.  Sometimes  the  shelter,  instead 
of  being  in  an  open  space  on  the  ground,  is  built  on  the  roof  or 
against  the  north  wall  of  a  building,  or  outside  of  one  of  the 
windows.  Fig.  2  shows  an  ordinary  shelter. 

A  still  simpler  method  of  exposure  is  described  in  the  "  In- 
structions for  Voluntary  Observers  "  (United  States  Weather 


14 


INSTRUMENTAL    OBSERVATIONS. 


Bureau,  1892)  as  follows  :  "  Select  a  north  window,  prefer- 
ably of  an  unoccupied  room,  especially  in  winter.  Fasten  the 
blinds  open  at  right  angles  to  the  wall  of  the  house.  Fasten  a 

narrow  strip  3  inches 
wide  across  the  window 
outside,  and  from  8  to 
12  inches  from  the 
window-pane.  To  this 
fasten  the  thermom- 
eters." If  none  of 
these  methods  of  shel- 
tering the  instrument 
is  feasible,  the  ther- 
mometer may  be 
fastened  to  the  window 
frame,  about  a  foot 
from  the  window,  and 
so  arranged  that  it  can 
be  read  from  the  inside 
of  the  room  without 
opening  the  window. 
Readings  of  the  thermometer,  to  the  nearest  degree  of  tem- 
perature indicated  on  the  scale  by  the  top  of  the  mercury 
column,  are  to  be  made  at  the  regular  observation  hours,  and 
are  to  be  entered  in  your  record  book.  Temperatures  below 
zero  are  preceded  by  a  minus  sign  (— ).  A  table  similar  to 
that  suggested  towards  the  close  of  the  last  chapter  (p.  8) 
may  be  used  in  keeping  these  instrumental  records,  except  that 
actual  thermometer  readings  can  now  be  entered  in  the  column 
headed  "  Temperature,"  instead  of  using  only  the  general  terms 
warm,  cold,  chilly,  etc.  This  is  a  great  gain.  You  will  now  be 
able  to  give  fairly  definite  answers  to  many  of  the  questions 
asked  in  Chapter  I.  Answer  these  questions  with  the  help 
of  your  thermometer  readings,  as  fully  as  you  can. 


THE    WIND    VANE.  15 

The  greater  part  of  the  Temperate  Zone,  in  which  we  live,  is 
peculiar  in  having  frequent  and  rapid  changes  of  temperature,  not 
only  from  season  to  season,  but  from  day  to  day,  and  during  a 
single  day.  In  winter,  we  are  apt  to  have  a  warm  wind  immedi- 
ately after  a  spell  of  crisp  cold  weather.  In  summer,  cloudy,  cool 
days  come  as  a  sudden  relief  when  we  have  been  suffering  from 
intense  heat,  with  brilliant  sunshine. 

These  changes  give  a  variety  to  our  climate  which  is,  on  the 
whole,  very  beneficial  to  man.  The  North  Temperate  Zone,  with 
strong  seasonal  changes  in  temperature  and  weather,  is  the  zone  of 
the  highest  civilization  and  of  the  greatest  energy  of  man.  In  the 
Torrid  Zone  the  changes  of  temperature  are,  as  a  whole,  small. 
There  is  no  harsh  winter.  The  climate  is  monotonous  and  deaden- 
ing, rather  than  enlivening.  Man  finds  it  easy  to  live  without  much 
work,  and  the  inhabitants  of  the  Torrid  Zone  have  not,  as  a  rule, 
advanced  far  in  the  scale  of  civilization. 

The  wind  vane  used  by  the  Weather  Bureau  is  about  6  feet 
long,  and  has  a  divided  tail  made  of  pine  boards,  the  two 
pieces  making  an  angle  of  22i°.  The  purpose  of  this  divided 
tail  is  to  steady  the  vane  and 
to  make  it  more  sensitive  to 
light  currents. 

A  common  wind  vane  on  a 
neighboring  church  steeple 
or  flagstaff  will  usually  serve 
sufficiently  well  for  ordinary 
use.  Observations  of  wind 
direction  (to  eight  compass  FlG- 

points)  are  to  be  made  as  a  part  of  the  ordinary  weather 
record,  and  to  be  entered  in  the  .proper  column  of  the 
record  book. 

The  rain  gauge  consists  of  three  separate  parts,  the  receiver 
J.,  the  overflow  attachment  B,  and  the  measuring  tube  C. 

The  inside  diameter  of  the  top  of  the  receiver  in  the  standard 
Weather  Bureau  gauge  is  8  inches  (at  a  in  Fig.  4).  This 
receiver  has  a  funnel-shaped  bottom,  so  that  all  the  precipitation 


16 


INSTRUMENTAL    OBSERVATIONS. 


which  falls  into  it  is  carried  at  once  into  the  measuring  tube  <7, 
whose  inside  height  is  20  inches.  The  diameter  of  the  measur- 
ing tube  is  2.53  inches.  The  rain  falling  into  the  receiver  A 
fills  this  tube  0  to  a  depth  greater  than  the  actual  rainfall,  in 


Front  View. 


Vertical  Section. 


d\      e       d 


Receiver. 


Horizontal  Section,  E-F. 


17  18  18  20  21  22  23  24  Inches. 


Scale. 
FIG.  4. 

proportion  as  the  area  of  the  receiver  is  greater  than  the  area 
of  the  measuring  tube.  In  the  standard  Weather  Bureau 
gauges  the  ratio  of  the  area  of  the  receiver  to  the  area  of  the 
measuring  tube  is  such  that  the  depth  of  rainfall  is  magnified 
exactly  ten  times.  The  object  of  magnifying  the  amount  in 
this  way  is  to  measure  a  very  small  quantity  more  easily.  The 
narrow  portion  of  the  receiver  [d]  fits  over  the  top  of  the  measur- 
ing tube,  holding  the  latter  firmly  in  place  and  preventing  any 
loss  of  rainfall.  An  opening,  e,  in  the  lower  portion  of  the 
receiver  [J],  just  on  a  level  with  the  top  of  the  measuring  tube, 
serves  as  an  escape  for  the  water  into  the  overflow  attachment 
.B,  in  case  the  rainfall  is  so  heavy  as  to  more  than  fill  the  tube. 
The  inside  diameter  of  the  overflow  attachment  is  the  same  as 
that  of  the  receiver  (8  inches),  as  will  be  seen  from  the  figure. 


RECORDS    OF    RAINFALL.  17 

The  rain  gauge  should  be  firmly  set  in  a  wooden  frame,  so 
arranged  that  the  overflow  attachment  can  readily  be  removed 
from  the  frame.  The  box  in  which  the  gauge  is  sent  out  by 
the  manufacturer  is  usually  designed  to  serve  as  a  permanent 
support  when  the  gauge  is  set  up.  The  best  exposure  for  the 
gauge  is  an  open  space  unobstructed  by  large  trees,  buildings, 
or  fences.  Fences,  walls,  or  trees  should  be  at  a  distance  from 
the  gauge  not  less  than  their  own  height.  If  an  exposure  upon 
the  ground  is  out  of  the  question,  the  gauge  may  be  placed  upon 
a  roof,  in  which  case  the  middle  of  a  flat  unobstructed  roof  is 
the  best  position. 

Records  of  Rainfall.  —  Every  rain  gauge  is  provided  with  a 
measuring  stick,  which  is  graduated  into  inches  and  hundredths. 
It  must  be  remembered  that  the  amount  of  rain  in  the  measur- 
ing tube  is,  by  the  construction  of  the  ordinary  gauge,  ten  times 
greater  than  the  actual  rainfall.  This  fact  need  not,  however, 
be  taken  into  account  by  the  observer,  for  the  numbers  used  in 
graduating  the  measuring  sticks  have  all  been  divided  by  10, 
and  therefore  they  represent  the  actual  rainfall.  The  gradua- 
tions on  the  stick  indicate  hundredths  of  an  inch,  and  should 
appear  in  the  record  as  decimals  (.10,  .20,  etc.).  Ten  inches  of 
water  in  the  measuring  tube  will  reach  the  mark  1.00  on  the 
stick  ;  thus  1.00  denotes  1  inch  and  zero  hundredths  of  rain. 
One  inch  of  water  in  the  tube  will  reach  the  .10  mark,  indicat- 
ing y1^-  of  an  inch.  The  shortest  lines  on  the  measuring  stick 
denote  successive  hundredths  of  an  inch.  Thus,  if  water  col- 
lected comes  to  a  point  halfway  between  the  .10  and  .20  lines, 
the  amount  is  .15  inch,  and  so  on.  In  measuring  rainfall,  the 
stick  is  lowered  through  the  bottom  of  the  receiver  into  the 
measuring  tube,  and  on  being  withdrawn  the  wet  portion  of 
the  stick  at  once  shows  the  depth  of  water  in  the  tube.  Care 
must  be  exercised  to  put  the  end  of  the  stick  where  the  number- 
ing begins  first  into  the  gauge,  and  to  pass  the  stick  through 
the  middle  of  the  tube.  After  each  observation  the  gauge 


18  INSTRUMENTAL    OBSERVATIONS. 

should  be  emptied  and  drained,  and  immediately  put  back  into 
place.  When  the  total  rainfall  more  than  fills  the  measuring 
tube,  i.e.,  exceeds  2  inches,  the  receiver  should  first  be  lifted  off 
and  the  tube  removed  with  great  care  so  as  not  to  spill  any 
water.  After  emptying  the  tube,  the  surplus  water  in  the 
overflow  attachment  must  be  poured  into  the  measuring  tube 
and  measured.  The  amount  of  rainfall  thus  found  is  to  be 
added  to  the  2  inches  contained  in  the  measuring  tube  in  order 
to  give  the  total  rainfall.  If  any  water  happens  to  be  spilled 
during  its  removal  from  the  overflow  attachment,  then  the 
amount  in  the  tube  will  be  less  than  2  inches,  and  it  must  be 
carefully  measured  before  the  latter  is  emptied. 

During  the  winter  season,  in  all  regions  where  snow  forms 
the  chief  part  of  the  precipitation,  the  only  portion  of  the  rain 
gauge  that  need  be  exposed  is  the  overflow  attachment.  The 
snow  which  falls  into  the  gauge  may  be  measured  by  first  melt- 
ing the  snow  and  then  measuring  the  water  as  rainfall.  About 
10  inches  of  snow  give,  on  the  average,  1  inch  of  water,  but 
the  ratio  varies  very  greatly  according  to  the  density  of  the 
snow.  Besides  the  measurement  of  the  melted  snow  collected 
in  the  gauge,  it  is  customary  to  keep  a  record  of  the  depth  of 
snowfall  in  inches,  as  measured  by  means  of  an  ordinary  foot  rule 
or  a  yardstick,  on  some  level  place  where  there  has  been  little 
or  no  drifting. 

Measurements  of  rain  and  snowfall  are  usually  made  once  a 
day,  at  8  P.M.,  and  also  at  the  end  of  every  storm.  Enter  the 
amounts  of  precipitation  in  the  column  of  the  table  headed 
"  Amount "  and  state  always  whether  it  is  rain  or  melted  snow 
that  you  have  measured.  When  there  has  been  no  precipi- 
tation since  the  last  observation,  an  entry  of  0.00  should  be 
made  in  the  column  of  the  record  book  devoted  to  "  Amount 
of  Precipitation."  When  the  amount  is  too  small  to  measure, 
the  entry  T  (for  Trace}  should  be  made. 

Continue  your  non-instrumental  record  of  the  time  of  begin- 


THE    MERCUKIAL    BAROMETER.  19 

ning  and  ending  of  the  precipitation  as  before.  Whenever  it  is 
possible,  keep  a  record  of  the  total  amount  of  precipitation  in 
each  storm,  noting  this  under  "  Remarks."  Try  to  answer  such 
questions  as  are  asked  in  Chapter  I  with  the  help  of  your  instru- 
mental record  of  the  rain  and  snowfall.  Note  what  depths  of 
snow  in  different  snowstorms  are  necessary,  when  melted,  to 
make  1  inch  of  water. 

The  Mercurial  Barometer,  —  Air  has  weight.  At  sea  level 
this  weight  amounts  to  nearly  15  pounds  on  every  square 
inch  of  surface.  Imagine  a  layer  of  water,  34  feet  deep, 
covering  the  earth.  The  weight  of  this  water  on  every,  square 
inch  of  surface  would  be  the  same  as  the  weight  of  the  air. 
Under  ordinary  circumstances  the  weight  of  the  air  is  not 
noticeable,  because  air  presses  equally  in  all  directions,  and  the 
pressure  within  a  body  is  the  same  as  that  outside  of  it.  On 
account  of  this  equal  pressure  in  all  directions,  we  speak  of  the 
pressure  of  the  air  instead  of  its  weight.  The  effects  of  the  air 
pressure  may  become  apparent  when  we  remove  the  air  from  a 
surface.  By  working  the  piston  of  a  pump  in  a  well  we  may 
remove  the  pressure  on  the  surface  of  the  water  in  the  tube  of 
the  pump.  When  this  is  done,  a  column  of  water  rises  in  the 
tube  until  the  top  of  this  column  is  about  34  feet  above  the 
level  of  the  rest  of  the  water  in  the  well.  The  pressure  of 
the  atmosphere  on  the  water  outside  of  the  tube  holds  up  this 
column  of  water  inside  the  tube. 

Galileo  (1564-1642)  first  taught  that  the  air  has  weight.  His 
pupil  Torricelli  went  a  step  further.  Torricelli  saw  that  the 
column  of  water,  held  up  by  the  pressure  of  the  air  in  the  tube 
of  the  pump,  must  exactly  balance  a  similar  column  of  air, 
reaching  from  the  surface  of  the  water  in  the  well  to  the  top 
of  the  atmosphere.  The  column  of  water,  in  other  words, 
exactly  replaces  this  column  of  air.  While  working  on  this 
subject,  Torricelli,  in  1643,  performed  the  following  experiment. 
He  filled  a  glass  tube,  about  3  feet  long  and  closed  at  one  end, 


20  INSTRUMENTAL    OBSERVATIONS. 

with  mercury.  After  filling  the  tube,  he  put  his  finger  over 
the  open  end  and  inverted  the  tube  over  a  vessel  containing 
mercury.  When  the  lower  end  of  the  tube  was  below  the 
surface  of  the  mercury  in  the  dish,  he  removed  his  finger.  At 
once  the  column  of  mercury  fell  in  the  tube  until  it  stood  at 
a  height  of  about  30  inches,  leaving  a  vacant  space  of  6  inches 
in  the  upper  part  of  the  tube.  This  space  has  since  been  known 
as  the  Torricellian  vacuum.  Torricelli  had  proved  what  he  had 
expected,  viz.,  that  the  height  of  the  column  of  liquid  which 
replaces  and  balances  an  air  column  of  the  same  size  varies 
with  the  weight  of  that  liquid.  It  takes  a  column  of  water 
34  feet  long  to  balance  a  similar  column  of  air.  It  takes  a 
column  of  mercury  only  30  inches  long  to  balance  a  similar 
column  of  air.  This,  as  Torricelli  correctly  explained,  is  due 
to  the  fact  that  mercury  is  so  much  (13^-  times)  heavier  than 
water.  The  column  of  water  weighs  just  the  same  as  the  col- 
umn of  mercury.  Each  column  exactly  balances  an  air  column 
of  similar  cross-section.  The  height  of  the  water  or  of  the 
mercury  is  a  measure  of  the  weight  or  pressure  of  the  air.  The 
greater  the  pressure  on  the  surface  of  the  water  in  the  well, 
the  higher  will  be  the  top  of  the  water  in  the  pump.  The 
greater  the  pressure  on  the  surface  of  the  mercury  in  the  basin, 
in  the  experiment  of  Torricelli,  the  higher  will  the  mercury 
column  stand  in  the  glass  tube.  Either  water  or  mercury  may 
be  used  as  the  liquid  in  the  barometer.  Otto  von  Guericke 
(1602-1686),  of  Magdeburg,  constructed  a  water  barometer 
about  36  feet  long,  which  he  attached  to  the  outside  wall  of 
his  house.  This  barometer  he  used  for  some  months,  and  made 
predictions  of  coming  weather  changes  by  means  of  it.  A  water 
barometer  is,  however,  a  very  unwieldy  thing  to  manage,  on 
account  of  the  great  length  of  its  tube.  Furthermore,  water 
barometers  cannot  be  used  in  any  countries  where  the  tem- 
peratures fall  to  freezing.  Mercury  is  the  liquid  universally 
employed  in  barometers.  It  is  so  heavy  that  only  a  small 


THE    MERCURIAL    BAROMETER.  21 

column  of  it  is  necessary  to  balance  the  atmospheric  pressure. 
Therefore  a  mercurial  barometer  is  portable.  Further,  mercury 
does  not  freeze  until  the  temperature  falls  to  40°  below  zero. 

Another  name  which  should  be  mentioned  in  connection  with 
the  barometer  is  that  of  Blaise  Pascal,  who  in  1648  fully  con- 
firmed Torricelli's  results.  Pascal  saw  that  if  the  mercury 
column  is  really  supported  by  the  weight  of  the  air,  the  height 
of  that  column  must  be  less  on  the  summit  of  a  mountain  than 
at  the  base,  because  there  is  less  air  over  the  top  of  the  moun- 
tain than  at  the  bottom,  and  therefore  the  weight  of  the  air 
must  be  less  at  the  summit.  To  prove  this,  he  asked  his 
brother-in-law  Perrier,  who  lived  at  Clermont,  in  France,  to 
carry  the  Torricellian  tube  up  the  Puy-de-Dchne,  a  mountain 
somewhat  over  3500  feet  high  in  Central  France.  This  Perrier 
did  on  Sept.  19,  1648,  and  he  found,  as  predicted  by  Pascal, 
that  the  mercury  fell  steadily  in  the  tube  as  he  went  up  the 
mountain,  and  that  at  the  top  of  the  mountain  the  column  of 
mercury  was  over  3  inches  shorter  than  at  the  base. 

The  pressure  of  the  atmosphere  is  a  weather  element  which, 
unlike  the  other  elements  already  considered,  cannot  be  observed 
without  an  instrument.  We  cannot,  under  ordinary  conditions 
at  sea  level,  determine  by  any  of  our  senses  whether  the  pressure 
is  rising  or  falling,  or  is  stationary.  The  pressure  on  the  upper 
floors  of  one  of  our  high  buildings  is  shown  by  a  barometer  to 
be  considerably  lower  than  it  is  at  the  level  of  the  street  below, 
and  yet  we  notice  no  difference  in  our  feelings  at  the  two 
levels. 

It  is  only  when  we  ascend  far  into  the  air,  as  in  climbing  a 
high  mountain  or  in  a  balloon,  that  the  much-diminished  pres- 
sure at  these  great  heights  perceptibly  influences  the  human 
body.  Mountain  climbers  and  aeronauts  who  reach  altitudes 
of  15,000  to  20,000  feet  or  more,  usually  suffer  from  headache, 
nausea,  and  faintness,  which  have  their  cause  in  the  reduced 
pressure  encountered  at  these  heights. 


22  INSTRUMENTAL    OBSERVATIONS. 

The  ordinary  mercurial  barometer  in  use  to-day  is,  essentially, 
nothing  more  than  the  glass  tube  and  vessel  of  Torricelli's 
famous  experiment.  A  simple  form  of  the  mercurial  barometer 
is  shown  in  Fig.  5.  It  consists  of  a  glass  tube  about  one-quarter 
of  an  inch  in  inside  diameter  and  about  36  inches  long.  This 
tube,  closed  at  the  top  and  open  at  the  bottom,  is  filled  with 
mercury,  the  lower,  open  end  dipping  into  a  cup 
of  mercury  known  as  the  cistern.  The  space  above 
the  mercury  is  a  vacuum.  The  mercury  extends 
inside  the  tube  to  a  height  corresponding  to  the 
weight  or  pressure  of  the  air,  the  vertical  height  of 
the  top  of  the  mercury  column  above  the  level  of 
the  mercury  in  the  cistern,  in  inches  and  hundredths 
of  an  inch,  being  the  barometer  reading.  At  sea 
level  the  normal  barometer  reading  is  about  80 
inches.  There  is  an  opening  near  the  top  of  the 
cistern,  at  the  back  of  the  instrument,  through  which 
the  air  gains  access  to  the  mercury  and  holds  up 
the  mercury  column.  It  will  readily  be  seen  that, 
as  the  mercury  in  the  tube  rises,  the  level  of  the 
mercury  in  the  cistern  falls,  and  vice  versa,  so  that 
there  is  a  varying  relation  between  the  two  levels. 
In  order  to  have  the  reading  accurate,  it  is  necessary 
that  the  surface  of  the  mercury  in  the  cistern  should 
be  just  at  the  zero  of  the  barometer  scale  when  a 
reading  is  made.  To  accomplish  this,  the  bottom  of 
the  cistern  consists  of  a  buckskin  bag  which  may  be 
raised  or  lowered  by  means  of  a  thumb-screw,  seen 
at  the  lower  end  of  the  instrument.  The  level  of 

r  IG.  5. 

the  mercury  may  thus  be  changed  and  adjusted  to 
the  top  of  a  black  line,  marked  on  the  outside  of  the  cistern, 
and  which  indicates  the  zero  of  the  scale.  Before  making  a 
reading,  the  surface  of  the  mercury  in  the  cistern  must  be  raised 
or  lowered  until  it  just  reaches  this  black  line.  Then  the  top 


THE    ANEROID    BAROMETER. 


23 


of  the  mercury  column  will  give  the  pressure  of  the  air.  The 
reading  is  made  on  an  aluminum  scale  at  the  top  of  the  wooden 
back  on  which  the  tube  is  mounted,  this  scale  being  graduated 
both  on  the  English  and  on  the  metric  system.  This  barometer 
may  be  hung  against  the  wall  of  a  room. 

The  aneroid  barometer  (Greek:  without  fluid),  although 
less  desirable  in  many  ways  than  the  mercurial,  is  nevertheless  a 
useful  instrument  for  rough  observations.  The  aneroid  is  not 
good  for  careful  scientific  work,  because  its  readings  are  apt  to 
be  rather  inaccurate.  To  be  of  much  value  in  indicating  exact 
pressures,  it  should  frequently  be 
compared  with  and  adjusted  to  a 
mercurial  barometer.  An  ordi- 
nary aneroid  barometer  is  shown 
in  Fig.  6. 

In  this  instrument  the  changes 
in  atmospheric  pressure  are 
measured  by  their  effects  in  alter- 
ing the  shape  of  a  small  metallic 
box,  known  as  the  vacuum 
chamber.  The  upper  and  lower 
surfaces  of  this  box  are  made 
of  thin  circular  sheets  of  corru- 
gated German  silver,  soldered  together  around  their  outer  edges, 
thus  forming  a  short  cylinder.  From  this  the  air  is  exhausted, 
and  it  is  then  hermetically  sealed.  A  strong  steel  spring,  inside 
or  outside  of  the  vacuum  chamber,  holds  apart  the  corrugated 
surfaces,  which  tend  to  collapse,  owing  to  the  pressure  of  the 
external  air  upon  them.  An  increase  or  decrease  in  the  air 
pressure  is  accompanied  by  an  approach,  or  a  drawing  apart, 
of  the  surfaces  of  the  chamber.  These  slight  movements  are 
magnified  by  means  of  levers,  a  chain,  and  a  spindle,  and  are 
made  to  turn  an  index  hand  or  pointer  on  the  face  of  the 
instrument.  The  outer  margin  of  the  face,  underneath  the 


24  INSTRUMENTAL    OBSERVATIONS. 

glass,  is  graduated  into  inches  and  hundredths,  and  the  pressure 
may  thus  be  read  at  once. 

As  the  tension  of  the  steel  spring  varies  with  the  tempera- 
ture, aneroids  are  usually  compensated  for  temperature  by  hav- 
ing one  of  the  levers  made  of  two  different  metals,  e.g.,  brass 
and  iron,  soldered  together,  or  else  by  leaving  a  small  quantity 
of  air  in  the  vacuum  chamber.  This  air,  when  heated, 
expands,  and  thus  tends  to  compensate  for  the  weaker 
action  of  the  spring,  due  to  the  higher  temperature.  At 
best,  however,  this  compensation  is  but  imperfect,  and  this 
fact,  together  with  the  friction  of  the  different  parts,  the 
changes  in  the  spring  with  age,  and  the  need  of  frequent 
adjustments,  makes  aneroids  rather  inaccurate.  They  may 
be  adjusted  to  mercurial  barometers  by  means  of  a  small 
screw,  whose  head  may  be  found  on  the  lower  surface  of  the 
instrument.  The  words  fair,  stormy,  etc.,  which  frequently 
appear  on  the  face  of  aneroid  barometers,  are  of  little  use  in 
foretelling  weather  changes,  as  no  definite  pressures  always 
occur  with  the  same  weather  conditions.  The  instrument 
should  be  tapped  lightly  a  few  times  with  the  finger  before 
a  reading  is  made.  The  second  pointer,  which  is  often  found 
in  aneroids,  is  set  by  the  observer  on  the  position  marked  by 
the  index  hand  when  he  makes  his  reading.  The  difference 
between  the  pressure  marked  by  this  set  pointer  and  that 
shown  by  the  index  hand  at  the  next  observation  is  the  meas- 
ure of  the  change  of  pressure  in  the  interval. 

Another  column  must  now  be  added  to  the  record  book 
(preferably  between  the  columns  devoted  to  temperature  and 
wind)  to  receive  the  "  Pressure  in  Inches  and  Hundredths." 

Is  the  pressure  constant  (i.e.,  are  the  readings  always  the 
same)  or  does  it  vary?  If  it  varies,  is  there  any  apparent 
system  in  the  variations  ?  Is  there  a  tendency  to  a  daily  maxi- 
mum ?  To  a  daily  minimum  ?  If  so,  about  what  time  do  these 
occur,  respectively  ?  What  is  the  average  variation  (in  inches 


SUMMAKY    OF    OBSERVATIONS.  25 

and  hundredths)  in  the  course  of  a  day  ?  What  is  the  greatest 
difference  in  pressure  which  you  have  observed  in  a  day? 
What  is  the  least  ?  Does  the  pressure  seem  to  vary  more  or 
less  in  the  colder  months  than  in  the  warmer  ?  Has  the  height 
of  the  mercury  column  any  relation  to  the  weather?  Are  we 
likely  to  have  rainy  weather  with  rising  barometer?  Is  the 
velocity  of  the  wind  related  to  the  pressure  in  any  way? 
How  ?  Can  you  make  any  general  rules  for  weather  prediction 
based  on  the  action  of  the  barometer  ?  What  rules  ? 

Tabulation  of  Observations.  —  The  tables  suggested  in  the  pre- 
ceding chapter  can  be  used  unchanged  with  the  simple  instru- 
ments just  described. 

Summary  of  Observations.  —  At  the  end  of  each  month  sum- 
marize your  instrumental  observations  in  the  following  way  :  — 

Temperature.  —  Add  together  all  your  temperature  readings  ; 
divide  their  sum  by  the  total  number  of  observations  of  tem- 
perature, and  the  quotient  will  give  you  a  sufficiently  accurate 
mean  or  average  temperature  for  the  month  in  question.  It  is 
to  be  noted  that  the  mean  monthly  temperatures  obtained  from 
these  observations  will  be  much  more  accurate  if  the  thermome- 
ter readings  are  made  at  7  A.M.  and  7  P.M.,  at  8  A.M.  and  8  P.M., 
etc.,  and  the  mean  of  these  is  taken  ;  or  if  the  mean  is  derived 
from  the  maximum  and  the  minimum  temperatures,  discussed 
in  Chapter  III.  This  mean  temperature  should  be  written  at  the 
bottom  of  the  temperature  column,  and  marked  "  Mean."  The 
mean  monthly  temperature  is  one  of  the  important  meteoro- 
logical data  in  considering  the  climatic  conditions  of  any  place. 

Wind. — Determine  the  frequency  of  the  different  wind  direc- 
tions by  counting  the  total  number  of  times  the  wind  has  blown 
from  N.,  NE.,  E.,  etc.,  during  the  month.  The  wind  which 
you  have  observed  the  greatest  number  of  times  is  the  prevailing 
wind.  It  may,  of  course,  happen  that  two  or  three  directions 
have  been  observed  an  equal  number  of  times.  The  number  of 
calms  should  also  be  recorded. 


26 


INSTRUMENTAL    OBSERVATIONS. 


Rainfall.  —  The  total  monthly  precipitation  is  obtained  by 
adding  together  all  the  separate  amounts  of  rainfall  noted  in 
your  record  book,  and  expressing  the  total,  in  inches  and  hun- 
dredths,  at  the  bottom  of  the  rainfall  column.  You  now  have 
the  means  for  comparing  one  month's  rainfall  with  that  of 
another  month,  and  of  seeing  how  these  amounts  vary. 

Examine  carefully  also  your  non-instrumental  observations. 
See  whether  you  can  draw  any  general  conclusions  as  to  the 
greater  prevalence  of  cloud,  or  of  rain  or  snow,  in  one  month 
than  in  another.  Did  the  last  month  have  more  high  winds 
than  the  one  before  ?  Or  than  the  average  ?  Were  the  tem- 
perature changes  more  sudden  and  marked?  Was  there  more 
or  less  precipitation  than  in  previous  months  ? 


CHAPTER    III. 

ADVANCED   INSTRUMENTAL   OBSERVATIONS. 

THE  instruments  for  more  advanced  study  are  the  following  : 
maximum  and  minimum  thermometers,  wet  and  dry  bulb  ther- 
mometers, sling  psychrometer,  standard  barometer,  thermograph, 
barograph,  and  anemometer. 

Maximum  and  minimum  thermometers  are  usually  mounted 
together  on  a  board,  as  shown  in  Fig.  7,  the  lower  one  of  the 


FIG.  7. 


two  being  the  maximum,  and  the  upper  the  minimum.  In  the 
view  of  the  instrument  shelter  (Fig.  2),  these  thermometers  are 
seen  on  the  left.  The  minimum  thermometer,  when  attached 


MAXIMUM    AND    MINIMUM    THERMOMETERS.  27 

to  its  support,  is  either  exactly  horizontal  or  else  slopes  down- 
ward somewhat  towards  the  bulb  end,  as  shown  in  Fig.  7. 
These  instruments,  as  their  names  imply,  register  the  highest 
and  the  lowest  temperatures,  respectively,  which  occur  during 
each  day  of  24  hours.  The  maximum  thermometer  is  filled 
with  mercury.  Its  tube  is  narrowed  just  above  the  bulb,  in 
such  a  way  that  the  mercury  passes  through  the  constriction 
with  some  difficulty.  As  the  temperature  rises,  the  mercury, 
in  expanding,  is  forced  out  from  the  bulb  through  this  narrow 
passage.  When  the  temperature  falls,  however,  the  mercury 
above  this  point  cannot  get  back  into  the  bulb,  there  being 
nothing  to  force  it  back.  The  length  of  the  mercury  column, 
therefore,  remains  the  same  as  it  was  when  the  temperature  was 
highest,  and  the  instrument  is  read  by  observing  the  number  of 
degrees  indicated  by  the  top,  or  right-hand  end,  of  the  mercury 
column  upon  the  scale.  After  reading,  the  thermometer  is  set 
by  removing  the  brass  pin  upon  which  the  bulb  end  rests,  and 
whirling  the  instrument  rapidly  around  the  pin  to  which  its 
upper  end  is  fastened.  By  this  process  the  mercury  is  driven 
back  into  the  bulb,  past  the  constriction.  Care  must  be  taken 
to  stop  the  thermometer  safely  while  it  is  whirling.  After  set- 
ting, the  reading  of  the  maximum  thermometer  should  agree 
closely  with  that  of  the  ordinary  or  dry-bulb  thermometer. 

The  minimum  thermometer  is  filled  with  alcohol,  and  contains 
within  its  tube  a  small  black  object,  called  the  index,  which 
resembles  a  double-headed  black  pin.  The  instrument  is  so 
constructed  that  this  index,  when  placed  with  its  upper,  or 
right-hand  end,  at  the  surface  of  the  alcohol,  is  left  behind, 
within  the  alcohol,  when  the  temperature  rises.  On  the  other 
hand,  when  the  temperature  falls,  the  index  is  drawn  towards 
the  bulb  by  the  surface  cohesion  of  the  alcohol,  the  top  or  right 
end  of  the  index  thus  marking  the  lowest  temperature  reached. 
The  upper  end  of  the  thermometer  is  firmly  fastened,  by  means 
of  a  screw,  to  a  brass  support,  while  the  lower  end  rests  upon  a 


28 


INSTRUMENTAL    OBSERVATIONS. 


notched  arm.  In  setting  this  instrument,  the  bulb  end  is  raised 
until  the  index  slides  along  the  tube  to  the  end  of  the  alcohol 
column.  The  thermometer  is  then  carefully  lowered  back  into 
the  notch  just  referred  to.  Maximum  and  minimum  thermom- 
eters need  to  be  read  only  once  a  day,  in  the  evening.  The 
temperatures  then  recorded  are  the  highest  and  lowest  reached 
during  the  preceding  24  hours.  The  observation  hour  is  pref- 
erably 8  P.M.,  but  if  this  is  inconvenient,  or  impracticable, 
the  reading  may  be  made  earlier  in  the  afternoon.  The  hour, 
however,  should  be  as  late  as  possible,  and  should  not  be  varied 
from  day  to  day.  The  maximum  temperature  sometimes  occurs 
in  the  night.  The  maximum  and  the  minimum  temperatures 
should  be  entered  every  day,  in  a  column  headed  "  Maximum 
and  Minimum  Temperatures,"  in  your  record  book. 

The  wet  and  dry  bulb  thermometers,  together  commonly 
known  as  the  psychrometer  (Greek  : 
cold  measure),  are  simply  two  ordinary 
mercurial  thermometers,  the  bulb  of 
one  of  which  is  wrapped  in  muslin, 
and  kept  moist  by  means  of  a  wick 
leading  from  the  muslin  cover  to  a 
small  vessel  of  water  attached  to  the 
frame  (see  Fig.  8).  The  wick  carries 
water  to  the  bulb  just  as  a  lamp  wick 
carries  oil  to  the  flame.  The  psychrom- 
eter is  seen  inside  the  shelter  on  the 
right  in  Fig.  2. 

The  air  always  has  more  or  less 
moisture  in  it.  Even  the  hot,  dry  air 
of  deserts  contains  some  moisture. 
This  moisture  is  either  invisible  or 
visible.  When  invisible  it  is  known 
as  water  vapor,  and  is  a  gas.  When 
visible,  it  appears  as  clouds  and  fog, 


FIG.  8. 


WET  AND  DRY  BULB  THERMOMETERS.          29 

or  in  the  liquid  or  solid  form  of  rain,  snow,  and  hail.  The 
amount  of  moisture  in  the  air,  or  the  humidity  of  the  air, 
varies  according  to  the  temperature  and  other  conditions. 
When  the  air  contains  as  much  water  vapor  as  it  can  hold, 
it  is  said  to  be  saturated.  Its  humidity  is  then  high.  When 
the  air  is  not  saturated,  evaporation  goes  on  into  it  from  mo'ist 
surfaces  and  from  plants.  Water  which  changes  to  vapor  is 
said  to  evaporate. 

This  process  of  evaporation  needs  energy  to  carry  it  on,  and 
this  energy  often  comes  from  the  heat  of  some  neighboring 
body.  When  you  fan  yourself  on  a  very  hot  day  in  summer, 
the  evaporation  of  the  moisture  on  your  face  takes  away  some 
of  the  heat  from  the  skin,  and  you  feel  cooler.  The  drier  the 
air  on  a  hot  day,  the  greater  is  the  evaporation  from  all  moist 
bodies,  and  hence  the  greater  the  amount  of  cooling  of  the 
surfaces  of  those  bodies.  For  this  reason  a  hot  day  in  summer, 
when  the  air  is  comparatively  dry,  that  is,  not  saturated  with 
moisture,  is  cooler,  other  things  being  equal,  than  a  hot  day 
when  the  air  is  very  moist.  Over  deserts  the  air  is  often  so 
hot  and  dry  that  evaporation  from  the  face  and  hands  is  very 
great,  and  the  skin  is  burned  and  blistered.  Over  the  oceans, 
near  the  equator,  the  air  is  hot  and  excessively  damp,  so  that 
there  is  hardly  any  cooling  of  the  body  by  evaporation,  and  the 
conditions  are  very  uncomfortable.  This  region  is  known  as 
the  "  Doldrums." 

The  temperatures  that  are  felt  at  the  surface  of  the  skin,  espe- 
cially where  the  skin  is  exposed,  as  on  the  face  and  hands,  have 
been  named  sensible  temperatures.  Our  sense  of  comfort  in  hot 
weather  depends  on  the  sensible  temperatures.  These  sensible 
temperatures  are  not  the  same  as  the  readings  of  the  ordinary 
(dry-bulb)  thermometer,  because  our  sensation  of  heat  or  cold 
depends  very  largely  on  the  amount  of  evaporation  from  the 
surface  of  the  body,  and  the  temperature  of  evaporation  is  ob- 
tained by  means  of  the  wet-bulb  thermometer.  Wet-bulb  read- 
ings at  the  various  stations  of  the  Weather  Bureau  are  entered  on 


30  INSTRUMENTAL    OBSERVATIONS. 

all  our  daily  weather  maps.  In  summer  (July)  the  sensible  (wet- 
bulb)  temperatures  are  20°  below  the  ordinary  air  temperature  in 
the  dry  southwestern  portion  of  the  United  States  (Nevada,  Ari- 
zona, Utah).  The  mean  July  sensible  temperatures  there  are  from 
50°  to  65°;  while  on  the  Atlantic  coast,  from  Boston  to  South 
Carolina,  they  are  between  65°  and  75°.  Hence  over  the  latter 
district  the  temperatures  actually  experienced  in  July  average 
higher  than  in  the  former. 

Unless  the  air  is  saturated  with  water  vapor,  the  evaporation 
from  the  surface  of  the  wet-bulb  thermometer  will  lower  the 
temperature  indicated  by  that  instrument  below  that  shown  by 
the  dry-bulb  thermometer  next  to  it,  from  which  there  is  no 
evaporation.  The  drier  the  air,  the  greater  the  evaporation, 
and  therefore  the  greater  the  difference  between  the  readings 
of  the  two  thermometers.  By  means  of  tables,  constructed  on 
the  basis  of  laboratory  experiments,  we  may,  knowing  the  read- 
ings of  the  wet  and  dry  bulb  thermometers,  easily  determine 
the  dew-point  and  the  relative  humidity  of  the  air  —  important 
factors  in  meteorological  observations  (see  Chapter  XXVI).  In 
winter,  when  the  temperature  is  below  freezing,  the  muslin  of 
the  wet-bulb  thermometer  should  be  moistened  with  water  a 
little  while  before  a  reading  is  to  be  made.  The  amount  of 
water  vapor  which  air  can  contain  depends  on  the  temperature 
of  the  air.  The  higher  the  temperature,  the  greater  is  the 
capacity  of  the  air  for  water  vapor.  Hence  it  follows  that, 
if  the  temperature  is  lowered  when  air  is  saturated,  the  capacity 
of  the  air  is  diminished.  This  means  that  the  air  can  no  longer 
contain  the  same  amount  of  moisture  (invisible  water  vapor)  as 
before.  Part  of  this  moisture  is  therefore  changed,  condensed, 
as  it  is  said,  from  the  condition  of  water  vapor  into  that  of 
cloud,  fog,  rain,  or  snow.  The  temperature  at  which  this 
change  begins  is  called  the  dew-point  of  the  air. 

The\  relative  humidity  of  the  air  is  the  ratio  between  the 
amount  of  water  vapor  which  the  air  contains  at  any  particular 
time  and  the  total  amount  which  it  could  contain  at  the  tern- 


SLING    PSYCHROMETElt. 


31 


perature  it  then  has.  Relative  humidity  is  expressed  in  per- 
centages. Thus,  air  with  a  relative  humidity  of  50^  has  just 
half  as  much  water  vapor  in  it  as  it  could  hold. 

It  is  found  that  the  readings  of  the  wet-bulb  thermometer 
are  considerably  affected  by  the  amount  of  air  movement  past 
the  bulb,  and  that  in  a  light  breeze,  or  in  a  calm,  the  reading 
does  not  give  accurate  results  as  to  the  humidity  of  the  general 
body  of  air  outside  the  shelter. 

To  overcome  this  difficulty  another  form  of  psychrometer  has 
been  devised. 

The  sling  psychrometer  (Fig.  9)  consists  simply  of  a  pair 
of  wet  and  dry  bulb  thermometers,  fastened 
together  on  a  board  or  a  strip  of  metal,  to  the 
upper  part  of  which  a  cord  with  a  loop  at  the  end 
is  attached.  In  this  form  of  psychrometer  there 
is  no  vessel  of  water  and  no  wick,  but  the  muslin 
cover  of  the  wet-bulb  thermometer  must  be 
thoroughly  wet,  by  immersion  in  water,  just 
before  each  observation.  The  instrument  is  then 
whirled  around  the  hand  at  the  rate  of  about  12 
feet  a  second.  After  whirling  about  50  times, 
note  the  readings,  and  then  whirl  the  instrument 
again,  and  so  on,  until  the  wet  bulb  reaches  its 
lowest  reading.  The  lowest  reading  of  the  wet 
bulb,  and  the  reading  of  the  dry  bulb  at  the  same 
time,  are  the  two  observations  that  should  be 
recorded.  Take  care  to  have  the  muslin  wet 
throughout  each  observation,  and'  in  windy 
weather  stand  to  leeward  of  the  instrument,  so 
that  it  may  not  be  affected  by  the  heat  of  your  body.  The 
true  reading  may  be  obtained  within  two  or  three  minutes. 

Make  observations  with  the  wet-bulb  thermometer  or  the 
sling  psychrometer  as  a  part  of  your  regular  daily  weather 
record.  Note  the  temperatures  indicated  by  the  wet  and  dry 


FIG. 


FIG.  10. 


INSTRUMENTAL    OBSERVATIONS. 

bulbs,  and,  by  means  of  the  table  in  Chapter 
XXVI,  obtain  the  dew-point  and  the  relative  hu- 
midity of  the  air  at  each  observation.  Enter  these 
data  in  your  record  book,  in  a  column  headed 
"  Humidity,"  and  subdivided  into  two  columns, 
one  for  the  dew-point  and  one  for  the  relative 
humidity. 

By  means  of  observations  with  the  psychrometer 
you  will  be  able  to  answer  such  questions  as  the 
following :  - 

Does  the  relative  humidity  vary  from  day  to  day? 
Has  it  any  relation  to  the  direction  of  the  wind? 
To  the  state  of  the  sky  ?  To  precipitation  ?  Does 
it  show  any  regular  variations  during  the  course  of 
a  day  ?  How  does  a  high  degree  of  relative  hu- 
midity affect  you  in  cold  weather?  In  hot  weather? 
Between  what  limits  of  percentages  does  the  rela- 
tive humidity  vary  ?  Do  the  changes  come  gradu- 
ally or  suddenly?  Are  these  changes  related  in 
any  way  to  the  changes  in  the  other  weather  ele- 
ments? How  do  the  sensible  temperatures  vary? 
In  what  weather  conditions  do  the  sensible  tem- 
peratures differ  most  from  the  air  temperatures  ? 
In  what  seasons  ?  Compare  the  sensible  tempera- 
tures obtained  by  your  own  observations  with  the 
sensible  temperatures  at  various  stations  of  the 
Weather  Bureau,  as  given  on  the  daily  weather 
map.  Are  there  any  fairly  regular  differences 
between  the  sensible  temperatures  observed  at  your 
own  station  and  the  Weather  Bureau  stations  ? 

Standard  Mercurial  Barometer.  —  A  simple  form 
of  barometer  has  been  described  in  Chapter  II. 
The  ordinary  standard  mercurial  barometer  used 
bv  the  Weather  Bureau  (Fig.  10)  has  the  glass 


STAND  A11D    MERCURIAL    BAROMETEK.  33 

tube  containing  the  mercury  surrounded  by  a  thin  brass 
covering,  through  which  openings  are  cut,  near  the  top,  on 
the  front  and  back,  exposing  to  view  the  glass  tube  and  the 
top  of  the  mercury  column.  On  one  side  of  this  opening  there 
is  a  strip  of  metal,  graduated  to  inches  and  tenths  or  twentieths, 
by  means  of  which  the  height  of  the  barometer  is  determined. 
This  strip,  for  barometers  used  at  or  near  sea  level,  is  about 
4  inches  long,  the  variations  in  pressure  under  normal  condi- 
tions not  exceeding  that  amount.  In  addition  to  this  fixed 
scale  there  is  a  small  scale,  also  graduated,  which  may  be 
moved  up  and  down  the  opening  in  the  enclosing  brass  case 
by  means  of  a  milled  head  outside  and  a  small  rack  and  pinion 
inside  the  brass  case.  This  movable  scale,  known  as  the  vernier 
from  the  name  of  its  inventor,  Vernier,  is  an  ingenious  device, 
by  means  of  which  more  accurate  readings  of  the  barometer  can 
be  made  than  is  possible  with  the  ordinary  fixed  scale.  A  vernier 
graduated  into  twenty-five  parts  enables  the  observer  to  make 
readings  accurately  to  the  one-thousandth  part  of  an  inch.  On 
the  front  of  the  barometer  there  is  a  small  thermometer,  known 
as  the  attached  thermometer.  The  bulb  of  this  thermometer, 
concealed  within  the  metal  casing  of  the  barometer,  is  nearly 
in  contact  with  the  glass  tube  containing  the  mercury.  The 
air,  upon  whose  weight  the  height  of  the  mercury  column 
depends,  gains  access  to  the  top  of  the  cistern  through  leather 
joints,  by  which  the  cistern  is  joined  to  the  glass  tube. 

Mercurial  barometers  of  the  Weather  Bureau  pattern  are  best 
hung  in  a  barometer  box,  fastened  securely  against  the  wall  of 
a  room,  where  there  is  a  good  light  on  the  instrument  and 
where  the  temperature  is  as  constant  as  possible. 

In  all  accurate  work  certain  corrections  have  to  be  applied  to 
barometer  readings  to  make  them  strictly  comparable.  These 
are:  (1)  correction  for  altitude;  (2)  correction  for  temperature; 
and  (3)  correction  for  latitude.  The  first  is  necessary  because 
of  the  fact  that  the  weight  of  the  air  decreases  upwards,  and 


34 


INSTRUMENTAL    OBSERVATIONS. 


a  barometer  reading  on  a  hill  or  a  mountain  is  not  comparable 
with  one  at  sea  level  unless  the  former  has  been  corrected  by 
the  addition  of  the  weight  of  the  column  of  air  between  the  hill 
or  mountain  and  sea  level.  The  correction  for  temperature  is 
rendered  necessary  by  the  fact  that  with  increasing  temperature 
the  mercury  in  the  barometer  tube  expands  more  than  the 
metallic  scale,  because  mercury  is  more  sensitive  to  heat,  and 
unless  some  allowance  is  made  for  this  fact,  barometer  readings 
made  at  high  temperatures  will  show  somewhat  too  high  a 
pressure.  The  readings  of  the  attached  thermometer  give  the 
temperature  of  the  mercury  and  are  used  in  making  the  correc- 
tions for  temperature.  As  gravity  varies  from  a  maximum 
value  at  the  poles  to  a  minimum  value  at  the  equator,  barom- 
eter readings  made  at  different  latitudes  are  corrected  for  lati- 
tude, which  means  that  they  are  reduced  to  latitude  45°,  midway 
between  0°  and  90°.  The  correction  is  +  0.08"  at  the  poles  and 
—  0.08"  at  the  equator.  Tables  for  use  in  correcting  barometer 
readings  for  altitude  and  for  temperature  are  given  in  Chapter 
XXVI. 


FIG.  11. 


Thermograph  and  Barograph.  —  Two  instruments  of  much 
interest  are  the  self-recording  thermometer,  or  thermograph,  and 
the  self-recording  barometer,  or  barograph,  manufactured  by 


THERMOGRAPH  AND  BAROGRAPH.  35 

Richard  Brothers  of  Paris.  In  the  thermograph  (Fig.  11)  there 
is  a  brass  cylinder  around  which  a  sheet  of  paper  is  wound,  this 
paper  being  divided  into  two-hour  intervals  of  time  and  into 
spaces  representing  differences  of  5°  or  10°  of  temperature.  The 
cylinder  revolves  once  in  a  week,  being  driven  by  clock-work 
contained  within  it.  The  thermometer  consists  of  a  flat,  bent, 
hollow  brass  tube  containing  alcohol,  one  end  of  the  tube  being 
fastened  to  the  metallic  frame  seen  at  the  right  of  the  figure, 
and  the  other  end  being  free  to  move.  With  rising  tempera- 
ture, the  liquid  in  the  tube  expanding  more  than  the  metallic 
casing,  by  reason  of  its  greater  sensitiveness  to  heat,  tends  to 
straighten  the  tube,  while  with  falling  temperature  the  elas- 
ticity of  the  tube  turns  it  into  a  sharper  curve.  These  move- 
ments of  the  free  end  of  the  tube  are  carried  through  a  train  of 
levers  and  thus  magnified.  At  the  end  of  the  last  lever  is  a 
metallic  pen  filled  with  ink,  which  rests  lightly  against  the 
paper  on  the  revolving  drum.  A  rise  or  fall  in  temperature  is 
thus  recorded  by  a  rise  or  fall  of  the  pen  on  the  record  sheet, 
and  a  continuous  curve  of  temperature  is  secured.  The  pen  of 
the  thermograph  should  be  frequently  adjusted  to  make  the 
reading  of  the  instrument  accord  with  that  of  a  standard  mer- 
curial thermometer,  and  care  should  be  taken  to  have  the  clock 
keep  good  time.  These  adjustments  can  readily  be  made  by 
means  of  a  screw  and  a  regulator,  respectively.  The  thermo- 
graph should  be  exposed  in  the  instrument  shelter  with  the 
other  thermometers.  The  sheets  should  be  changed,  the  clock 
wound,  and  the  pen  filled  once  a  week,  preferably  on  Monday, 
at  8  A.M.,  or  at  noon. 

The  continuous  records  written  by  a  thermograph  are  a 
valuable  addition  to  the  fragmentary  observations  which  are  the 
result  of  eye  readings  of  the  ordinary  thermometer.  From  the 
former  any  omitted  thermometer  readings  may  be  supplied.  The 
interest  of  thermograph  records  may  be  seen  in  the  following 
figure  (Fig.  12),  in  which  curves  traced  under  different  condi- 


36 


INSTRUMENTAL    OBSERVATIONS. 


tions  are  reproduced.  Curve  a  illustrates  a  period  of  clear 
warming  weather  at  Nashua,  N.  H.,  April  27-30,  1889.  Curve 
b  was  traced  during  a  spell  of  cloudy  weather  at  Nashua, 

6     Noon  6        12       6     Noon    6       12       6     Noon     6       12       6     Noon    6 


FIG.  12. 


accompanying  the  passage  of  a  West  India  hurricane,  Sept. 
13-16,  1889.  Curve  c  illustrates  the  change  from  a  time  of 
moderate  winter  weather  to  a  cold  spell  (Nashua,  Feb.  22-25, 
1889).  Curve  d  exhibits  a  steady  fall  of  temperature  from  the 
night  of  one  day  over  the  next  noon  to  the  following  night, 
during  the  approach  of  a  winter  cold  spell  (Nashua,  Jan.  19-21, 
1889).  Curve  e  shows  a  reverse  condition,  viz.,  a  continuous 
rise  of  temperature  through  a  night  from  noon  to  noon  (Nashua, 
Dec.  16-17,  1888).  Curve  /  shows  the  occurrence  of  a  high 
temperature  at  night,  caused  by  warm  southerly  winds,  fol- 
lowed by  cold  westerly  winds  (Cambridge,  Mass.,  Nov.  30- 
Dec.  1,  1890).  Curve  g  illustrates  the  sudden  rise  of  tempera- 
ture due  to  the  coming  of  a  hot,  dry  wind  (chinook)  at  Fort 
Assiniboine,  Mont.  (Jan.  19,  1892).  A  study  of  such  records 
leads  to  the  discovery  of  many  important  facts,  which  would 
be  completely  lost  sight  of  without  a  continuous  record. 

The  barograph  (Fig.  13)  is  very  similar  to  the  thermo- 
graph in  general  appearance.  The  essential  portion  of  this 
instrument  consists  of  a  series  of  six  or  eight  hollow  shells 
of  corrugated  metal  screwed  one  over  the  other  in  a  vertical 
column.  These  shells  are  exhausted  of  air,  and  form,  in 
reality,  an  aneroid  barometer  which  is  six  or  eight  times  as 


THERMOGRAPH  AND  BAROGRAPH. 


37 


sensitive  as  the  ordinary  single-chamber  aneroid.  The  springs 
for  distending  the  shells  are  inside.  The  base  of  the  column 
being  fixed,  the  upper  end  rises  and  falls  with  the  variations  in 


FIG.  13. 


pressure.  The  movements  of  the  shells  are  magnified  by  being 
carried  through  a  series  of  levers,  and,  as  in  the  thermograph, 
the  motion  is  finally  given  to  a  pen  at  the  end  of  the  long  lever. 
The  compensation  for  temperature  is  the  same  as  in  the  ordi- 
nary aneroid.  A  small  quantity  of  air  is  left  in  one  of  the 
shells  to  counteract,  by  its  own  expansion  at  increased  tem- 
perature, the  tendency  of  the  barometer  to  register  too  low  on 
account  of  the  weakening  of  the  springs.  The  barograph  may 
be  placed  upon  a  shelf  in  the  school-room,  where  it  can  remain 
free  from  disturbance,  and  yet  where  the  record  may  be  clearly 
seen.  The  general  care  of  the  barograph  is  the  same  as  that 
of  the  thermograph.  Brief  instructions  concerning  the  care 
and  adjustments  of  these  instruments  are  sent  out  by  the 
makers  with  each  instrument.  Frequent  comparison  with  a 
mercurial  barometer  is  necessary,  the  adjustment  of  the  baro- 
graph being  made  by  turning  a  screw,  underneath  the  column 
of  shells,  on  the  lower  side  of  the  wooden  case. 

Barograph  records  are  fully  as  interesting  as  those  made  by 
the  thermograph.     The  week's  record  traced  on   the  writer's 


38 


INSTRUMENTAL   OBSERVATIONS. 


barograph  during  a  winter 
voyage  from  Punta  Arenas, 
Strait  of  Magellan,  to 
Corral,  Chile,  Aug.  2-9, 
1897,  gives  a  striking  pic- 
ture of  the  rapid  and 
marked  changes  of  pressure 
during  seven  days  in  the 
South  Pacific  Ocean  (Fig. 
14). 

The  following  figure 
(Fig.  15)  presents  samples 
of  barograph  curves  traced 
at  Harvard  College  Observ- 
atory, Cambridge,  Mass., 
during  Feb.  22-28,  1887, 
and  May  17-23,  1887. 
The  February  curve  illus- 
trates well  the  large  and 
irregular  fluctuations  in 
pressure,  characteristic  of 
our  winter  months  ;  while 
the  May  curve  shows 
clearly  the  more  even  qual- 
ity of  the  pressure  changes 
in  our  summer. 

The  anemometer  shown 
in  Fig.  16  is  the  most 
generally  used  of  instru- 
ments designed  to  measure 
wind  velocity.  It  is  known 
as  the  Robinson  cup  ane- 
mometer, and  consists  of 
four  hollow  hemispherical 


ANEMOMETER. 


39 


cups  upon   arms  crossed   at  right  angles,  and  all  facing   the 
same     way     around    the    circle.     The     cross-arms    are    fixed 


1887 


May  is 


Feb.27 
May  22 


upon  a  vertical  axis  having  an  endless  screw  at  its  lower 
end.  When  the  cups  move  around,  the  endless  screw  turns 
two  dials  which  register  the  number  of  miles  traveled  by 


FIG.  16. 

the    wind.      The    Weather    Bureau    pattern    of    anemometer 
has  the  dials  mounted  concentrically,  the   outer  dial  having 


40  INSTRUMENTAL    OBSERVATIONS. 

100,  and  the  inner,  99  divisions.  The  revolutions  of  the 
outer  dial  are  recorded  on  the  inner  one,  and  in  making  an 
observation  of  the  number  of  miles  traveled  by  the  wind,  the 
hundreds  and  tens  of  miles  are  taken  from  the  inner  dial,  and 
the  miles  and  tenths  from  the  outer  one.  Take  from  the  inner 
scale  the  hundreds  and  tens  of  miles  contained  between  the 
zero  of  that  scale  and  the  zero  of  the  outer  one.  Take  on  the 
outer  scale  the  miles  and  tenths  of  miles  contained  between 
the  zero  of  that  scale  and  the  index  point  of  the  instrument. 
The  sum  of  these  readings  is  the  reading  of  the  instrument  at 
the  time  of  the  observation. 

Wind  velocities  are  recorded  in  miles  per  hour.  The 
velocity  of  the  wind  at  any  particular  moment  is  found  by 
noting  the  number  of  miles  and  tenths  of  miles  recorded  by 
the  index  before  and  after  an  interval  of  one  minute,  or  of 
five  minutes,  and  multiplying  this  rate  by  60  or  by  12  as  the 
case  may  be.  This  gives  the  number  of  miles  an  hour  that  the 
wind  is  blowing  at  the  time  of  observation. 

Records  of  wind  velocity  (in  miles  per  hour)  are  to  be  made 
at  each  regular  observation  hour,  and  are  to  be  entered  in  the 
proper  column  of  the  table  in  your  record  book.  The  total 
wind  movement  in  each  24  hours  is  to  be  observed  once  a 
day,  always  at  the  same  hour,  and  is  to  be  entered  in  its  proper 
column  in  the  record  book. 

The  total  wind  movement  for  24  hours  is  obtained  as  follows  : 
Subtract  the  reading  of  the  anemometer  at  12  noon  (or  8  A.M., 
or  any  other  hour)  of  the  preceding  day  from  the  reading  taken 
at  12  noon  or  the  corresponding  hour  of  the  current  day,  and 
the  difference  will  be  the  total  movement  of  the  wind.  When 
the  reading  of  the  anemometer  is  less  than  the  reading  of  the 
preceding  day,  990  miles  should  be  added  to  it ;  and  the 
remainder,  after  subtracting  the  reading  of  the  preceding  day, 
will  be  the  total  wind  movement  for  the  24  hours.  Thus  : 
To-day's  reading  =  91  miles  ;  yesterday's  reading  =  950  miles. 


NEPHOSCOPE.  41 

Hence  91  +  990  =  1081  miles,  1081  -  950  =  131  miles  =  total 
wind  movement  for  the  current  day. 

By  means  of  an  electrical  attachment  the  anemometer  may 
be  arranged  so  as  to  record  continuously  on  a  cylinder  rotat- 
ing by  clock-work,  a  pen  making  a  mark  on  the  paper  for 
every  mile  traveled  by  the  wind.  The  anemometer  should  be 
exposed  on  top  of  a  building  where  there  is  as  little  obstruc- 
tion as  possible  by  tall  chimneys,  higher  buildings,  and  the 
like. 

The  nephoscope  (Greek  :  cloud  observer)  is  an  instrument 
used  in  determining  the  directions  of  movement  of  clouds. 
These  directions,  if  determined  by  ordinary  eye  observation 
of  the  clouds  as  they  drift  across  the  sky,  are  apt  to  be  quite 
inaccurate.  The  best  method  of  observing  directions  of  cloud 
movement  is  to  note  the  path  of  the  reflection  of  the  cloud  in 
a  horizontal  mirror,  the  observer  looking  at  this  reflection 
through  an  eyepiece  which  remains  fixed  during  the  opera- 
tion. Such  a  horizontal  mirror,  adapted  to  measure  the  direc- 


FlG.  17. 


tion  of  motion  of  clouds,  is  known  as  a  nephoscope.  A  form 
of  nephoscope  devised  by  Mr.  H.  H.  Clayton,  of  Blue  Hill 
Observatory,  Hyde  Park,  Mass.,  is  shown  in  Fig.  17. 

This  instrument  consists  of  a  circular  mirror,  13  inches  in 


42  INSTRUMENTAL   OBSERVATIONS. 

diameter,  sunk  in  a  narrow  circular  wooden  frame,  on  top  of 
which  is  fastened  a  brass  circle,  S.W.N.E.,  divided  to  5°  of  arc. 
Inside  of  this  fixed  circle  is  a  movable  brass  one,  to  which  is 
attached  a  brass  arc,  ED,  rising  above  the  mirror  and  bearing 
a  movable  eyepiece,  0.  This  arc  forms  the  quadrant  of  a  circle 
whose  center  is  the  center  of  the  mirror,  and  is  divided  to  5° 
of  arc.  Its  top  is  held  vertically  over  the  center  of  the  mirror 
by  two  rods  fastened  to  the  movable  circle.  The  center  of  the 
mirror  A  is  marked  by  cross  lines  on  the  reflecting  surface,  the 
glass  of  which  is  thin.  In  order  to  determine  the  motion  of  a 
cloud,  the  movable  circle  and.  tripod  are  revolved  until  the  arc 
BD  is  in  the  vertical  plane  formed  by  the  cloud,  the  center  of 
the  mirror,  and  the  eye.  The  eyepiece  0  is  then  shifted  until 
some  point  of  the  cloud  image,  as  seen  through  the  eyepiece, 
is  projected  on  the  intersection  of  the  cross  lines  on  the  glass. 
The  cloud  image  soon  changes  its  position,  and  while  the  eye 
is  still  held  at  the  eyepiece,  a  small  index  is  placed  on  the  part 
of  the  cloud  image  which  previously  appeared  on  the  center  of 
the  mirror.  If  now  a  ruler  be  placed  on  the  index  and  the 
center  of  the  mirror  and  extended  backward,  its  intersection 
with  the  divided  scale  will  give  the  direction  from  which  the 
cloud  came  to  the  nearest  degree,  if  all  the  measurements  have 
been  accurately  made.  The  height  of  the  cloud  above  the 
horizon  is  found  by  reading  the  position  of  the  eyepiece  on 
the  divided  quadrant. 

The  nephoscope  may  be  placed  on  a  table,  out  of  doors 
in  fine  weather,  or  close  to  a  window  from  which  the  clouds 
to  be  observed  can  be  seen.  The  instrument  must  be 
properly  oriented,  so  that  the  four  points  marked  N.,  E., 
S.,  and  W.  on  the  frame  shall  correspond  to  the  four  chief 
compass  directions.  The  zero  (0°)  of  the  movable  brass 
scale  is  usually  put  at  the  S.  Hence,  if  a  cloud  is  found 
moving  from  exactly  SW.,  the  angular  measurement  of  its 
direction  of  motion  will  be  45°.  If  a  cloud  is  moving  from 


SUMMARY    OF    OBSERVATIONS.  43 

due  E.,  the  angular  measurement  of  its  direction  of  motion 
will  be  270°. 

When  the  sky  is  completely  overcast  with  a  uniform  layer 
of  cloud,  it  is  usually  impossible  to  determine  any  direction  of 
movement,  because  of  the  difficulty  of  selecting  and  keeping  in 
view,  on  the  mirror,  some  particular  point  of  cloud. 

Observations  with  the  nephoscope  may  be  made  as  often  as 
is  desired,  and  should  be  entered  in  an  appropriate  column  in 
the  record  book. 

Tabulation  of  Observations. — A  convenient  form  of  table  which 
may  be  used  in  the  complete  instrumental  observations  is  given 
on  the  next  page.  The  number  of  columns  and  their  arrange- 
ment may,  of  course,  be  varied  to  suit  the  number  and  the 
nature  of  the  records. 

Summary  of  Observations.  —  In  the  preceding  chapter  we  have 
seen  how  to  obtain  the  mean  monthly  temperature  from  the 
daily  observations,  the  frequency  of  the  different  wind  direc- 
tions for  each  month,  and  the  total  monthly  precipitation. 
The  addition  of  the  new  instruments,  the  maximum  and  mini- 
mum thermometers,  the  psychrometer,  the  anemometer,  and  the 
nephoscope,  enables  us  to  obtain  the  following  additional  data 
in  our  monthly  summaries. 

Temperature.  —  The  mean  monthly  temperature  maybe  obtained 
from  the  maximum  and  minimum  temperatures  as  follows  : 
Add  together  all  the  daily  maximum  and  minimum  tempera- 
tures for  a  month.  Divide  this  sum  by  the  total  number  of 
readings  you  have  made  of  each  thermometer  (i.e.,  one  reading 
of  the  maximum  and  one  of  the  minimum  each  day,  making 
two  readings  a  day),  and  the  result  will  be  the  mean  monthly 
temperature  derived  from  the  maximum  and  minimum  tempera- 
ture. This  is  a  more  accurate  mean  temperature  than  the  one 
noted  in  the  summary  of  the  preceding  chapter. 

Add  together  all  the  maximum  temperatures  noted  during 
one  month.  Divide  this  sum  by  the  number  of  observations, 


44 


INSTRUMENTAL    OBSERVATIONS. 


DATE. 

HOUR. 

TABLE  FOR  METEOROLOGICAL  RECORD. 

PRESSURE    (in 
inches). 

DRY  BULB. 

TEMPERATURE. 

WET  BULB. 

MAX. 

MIN. 

DEW- 
POINT. 

HUMIDITY. 

RELATIVE 
HUMIDITY. 

DIRECTION. 

4 

y, 
3 

VELOCITY 
(miles  per  hr.). 

TOTAL  MILES 
PER  DAY. 

KIND. 

CLOUDS. 

AMOUNT  (in 
tenths). 

DIRECTION  OF 
MOVEMENT. 

ANGULAR 
ALTITUDE. 

TIME  OF  BE- 
GINNING AND 
ENDING. 

PRECIPITATION. 

KIND. 

AMOUNT. 

R 
• 

SUMMARY   OF    OBSERVATIONS.  45 

and  the  result  gives  the  mean  maximum  temperature  for  the 
month. 

A  similar  operation  applied  to  the  minimum  temperatures 
gives  the  mean  minimum  temperature  for  the  month. 

In  meteorological  summaries  it  is  customary  also  to  include 
the  absolute  maximum  and  the  absolute  minimum  temperatures, 
i.e.,  the  highest  and  lowest  single  readings  of  the  thermometer 
made  during  each  month.  These  can  easily  be  determined  by 
simple  inspection  of  your  record  book.  Note  also  the  dates 
on  which  the  absolute  maximum  and  the  absolute  minimum 
occurred. 

The  absolute  monthly  range  of  temperature  is  the  difference, 
in  degrees,  between  the  absolute  maximum  and  the  absolute 
minimum. 

Humidity.  —  The  mean  relative  humidity  is  obtained  by  add- 
ing together  all  the  different  percentages  of  relative  humidity 
obtained  during  the  month,  and  dividing  this  sum  by  the  whole 
number  of  observations  of  this  weather  element. 

Wind.  —  The  mean  velocity  of  the  wind  corresponding  to  the 
different  wind  directions  is  readily  obtained  by  adding  together 
all  the  different  velocities  (in  miles  per  hour)  observed  in  winds 
from  the  different  directions,  and  dividing  these  sums  by  the 
number  of  cases.  The  wind  summaries  will  thus  give  the 
frequency  of  the  different  directions  during  each  month,  and 
the  corresponding  mean  velocities. 

The  maximum  hourly  wind  velocity  is  obtained  by  inspection 
of  the  velocity  column. 

The  total  monthly  wind  movement  is  readily  deduced  from  the 
daily  records  in  the  twelfth  column  of  the  table  on  p.  44. 

State  of  the  Sky.  —  In  connection  with  the  more  advanced 
records  described  in  this  chapter,  the  observations  of  cloudiness 
should  record  the  number  of  tenths  of  the  sky  cloudy,  as  closely 
as  the  amount  can  be  estimated  by  eye,  instead  of  indicating  the 
state  of  the  sky  as  cloudy,  fair,  etc.  A  detailed  record  of 


46  INSTRUMENTAL    OBSERVATIONS. 

cloudiness  in  tenths  gives  opportunity  to  determine  the  mean 
cloudiness  for  each  month,  by  averaging,  as  in  the  case  of  the 
other  means  already  described. 

If  nephoscope  observations  are  made,  the  monthly  summary 
may  include  the  mean  direction  of  cloud  movement  for  each 
month.  This  is  obtained  by  adding  together  all  the  different 
angular  measurements  of  directions  of  cloud  movement,  and 
dividing  by  the  whole  number  of  such  observations. 

By  means  of  your  monthly  summaries  compare  one  month 
with  another.  Notice  how  the  means  and  the  extremes  of  the 
different  weather  elements  are  related  ;  how  they  vary  from 
month  to  month.  Are  there  any  progressive  changes  in  temper- 
ature, cloudiness,  precipitation,  etc.,  from  month  to  month? 
What  are  the  changes  ?  Summarize,  in  a  short  written  state- 
ment, the  meteorological  characteristics  of  each  month  as  shown 
by  your  tables. 


PART  III.  —  EXERCISES  IN  THE  CONSTRUCTION  OF 
WEATHER  MAPS. 


CHAPTER   IV. 

THE   DAILY  WEATHER  MAP. 

THE  first  daily  weather  maps  were  issued  in  connection  with 
the  Great  Exhibition  of  1851  in  London.  The  data  were  col- 
lected by  the  Electric  Telegraph  Company  and  transmitted  to 
London  over  its  wires.  These  maps  were  published  and  sold 
daily  (excepting  Sundays)  from  Aug.  8  to  Oct.  11,  1851. 
The  first  official  weather  map  of  the  United  States  Weather 
Service  was  prepared  in  manuscript  on  Nov.  1,  1870,  and  on 
Jan.  14,  1871,  the  work  of  manifolding  the  maps  for  distribu- 
tion was  begun  at  Washington.  Previous  to  the  publication 
of  this  government  map,  Professor  Cleveland  Abbe  had  issued 
in  Cincinnati,  with  the  support  of  the  Chamber  of  Commerce 
of  that  city,  the  first  current  weather  maps  published  in  the 
United  States  (Feb.  24  to  Dec.  10,  1870).  In  France,  daily 
weather  maps  have  been  published  continuously  since  Sept. 
16,  1863. 

Two  things  are  essential  for  the  publication  of  a  daily  synop- 
tic weather  map  ;  first,  simultaneous  meteorological  observations 
over  an  extended  area  ;  and,  second,  the  immediate  collection 
of  these  observations  by  telegraph.  The  weather  map  of  the 
United  States  is  based  on  simultaneous  observations  made  at 
about  150  stations  in  different  parts  of  this  country,  besides 
several  cooperating  stations  in  Canada,  Central  America, 
Mexico,  and  the  West  Indies.  At  each  of  our  stations,  whose 

47 


48  CONSTRUCTION    OF    WEATHER    MAPS. 

location  may  be  seen  on  any  weather  map,  the  Weather  Bureau 
employs  one  or  more  observers,  who,  twice  a  day,  at  8  A.M. 
and  8  P.M.,  "  Eastern  Standard  Time,"  make  regular  observations 
of  the  ordinary  weather  elements,  i.e.,  temperature,  pressure, 
humidity,  wind  direction  and  velocity,  precipitation,  cloudi- 
ness, etc.  The  instruments  at  these  stations  are  all  standard, 
but  the  completeness  of  the  equipment  varies  according  to  the 
importance  of  the  station.  The  8  A.M.  observations  are  the 
only  ones  now  generally  used  in  the  preparation  of  weather 
maps.  When  the  Weather  Service  was  first  established,  tri- 
daily  charts  were  for  some  time  issued  from  the  central  office 
in  Washington.  On  April  1,  1888,  the  number  was  reduced 
to  two  a  day,  and  on  Sept.  30,  1895,  a  further  change  was 
made,  and  now  there  is  but  one  map  a  day. 

The  8  A.M.  observations,  as  soon  as  made,  are  corrected  for 
certain  instrumental  errors,  and  the  barometer  readings  are 
reduced  to  sea  level.  The  data  are  then  put  into  cipher,  not 
for  secrecy,  but  to  facilitate  transmission  and  to  lessen  the 
chances  of  error,  and  are  telegraphed  from  all  parts  of  the  coun- 
try to  the  central  office  of  the  Weather  Bureau  in  Washington. 
Besides  sending  their  own  messages  to  Washington,  all  the 
important  stations  of  the  Weather  Bureau  receive,  by  a  care- 
fully devised  system  of  telegraphic  circuits,  a  sufficient  number 
of  the  reports  from  other  stations  to  enable  their  observers  to 
draw  and  issue  local  weather  maps. 

The  observations  are  received  at  the  central  office  of  the 
Weather  Bureau  in  Washington  by  special  wires,  and  are 
usually  all  there  within  an  hour  after  the  readings  were  made. 
As  the  messages  are  received  in  the  forecast  room,  they  are 
translated  from  the  cipher  back  again  into  the  original  form, 
and  the  data  are  entered  upon  blank  maps.  The  official 
charged  with  making  the  forecasts  then  draws  upon  the  maps 
lines  of  equal  temperature,  lines  of  equal  pressure,  lines  of  equal 
pressure-change  and  temperature-change  during  the  past  24 


DAILY    WEATHER    MAP.  49 

hours.  These  several  sets  of  lines,  together  with  those  showing 
the  regions  of  precipitation  during  the  past  24  hours,  furnish 
the  necessary  data  on  which  the  forecasts  can  be  based.  In 
other  words,  the  forecast  official  has  before  him,  on  the  several 
maps,  a  bird's-eye  view  of  the  weather  conditions  over  the  United 
States  as  they  were  an  hour  before,  and  also  of  the  changes  that 
have  taken  place  in  these  conditions  during  the  preceding  24 
hours.  Thus,  by  knowing  the  general  laws  which  govern  the 
movements  of  areas  of  high  and  low  temperature,  of  fair  and 
stormy  weather,  across  the  country,  he  can  make  a  prediction  as 
to  the  probable  conditions  which  any  state  or  section  of  the 
country  will  experience  in  12,  24,  or  36  hours. 

In  a  later  chapter  some  suggestions  will  be  given  for  studies 
of  forecasting. 

The  forecasts  made  in  Washington,  and  printed  on  the 
Washington  daily  weather  map,  relate  to  all  sections  of  the 
United  States,  and  include  predictions  of  cold  waves,  killing 
frosts,  storm  winds,  river  floods,  and  the  like,  besides  the  ordi- 
nary changes  in  weather  conditions.  These  forecasts,  as  soon 
as  made,  are  at  once  given  to  the  local  newspapers  and  to  the 
press  associations.  They  are  also  sent  by  telegraph  to  all  regular 
stations  of  the  Weather  Bureau,  and  to  all  stations  at  which 
cautionary  or  storm  signals  are  to  be  displayed,  along  the 
Atlantic  or  Gulf  coasts,  and  on  the  Great  Lakes. 

The  Washington  weather  map  is  about  24  by  16  inches  in 
size,  and  is  newly  lithographed  each  day.  The  total  number 
of  maps  issued  from  the  central  office  during  the  fiscal  year 
ending  June  30,  1898,  was  310,250.  In  addition  to  these, 
there  are  now  84  stations  of  the  Weather  Bureau  in  different 
parts  of  the  country,  at  which  daily  weather  maps  are  issued 
and  local  forecasts  made.  These  latter  forecasts  are  made  by 
a  corps  of  local  forecast  officials,  each  of  whom  has  to  make 
the  weather  prediction  for  his  own  district.  At  first,  and  until 
within  a  few  years,  one  predicting  officer  in  Washington  made 


50  CONSTRUCTION    OF    WEATHER    MAPS. 

all  the  forecasts  for  the  country,  but  it  was  found  better 
to  have  the  country  divided  into  geographical  sections,  over 
each  one  of  which  the  meteorological  conditions  are  fairly  simi- 
lar, and  to  have  a  local  forecast  official  in  charge  of  each  section. 
These  local  forecast  officials  have  the  double  advantage  of  being 
able  to  study  the  weather  conditions  over  the  whole  country, 
as  sent  them  by  telegraph  each  morning,  and  also  of  knowing 
the  special  peculiarities  of  their  own  regions.  This  enables 
them  to  make  more  accurate  predictions  than  can  be  made  by 
an  official  who  may  be  one  or  two  thousand  miles  distant,  in 
Washington. 

The  greater  portion  of  the  maps  issued  at  the  map  stations 
outside  of  Washington  are  prepared  by  what  is  known  as  the 
chalk-plate  process,  suggested  by  Mr.  J.  W.  Smith,  local  fore- 
cast official  at  Boston.  This  process  is  as  follows:  A  thin 
covering  of  specially  prepared  chalk,  -J  of  an  inch  in  thickness, 
is  spread  upon  a  steel  plate  of  the  size  of  the  prospective 
weather  map.  On  this  chalk  are  engraved,  by  means  of  suit- 
able instruments,  the  various  weather  symbols,  the  lines  of 
equal  pressure  and  of  equal  temperature,  and  the  wind  arrows. 
The  plate  is  then  stereotyped  in  the  ordinary  way,  and  printed 
on  a  sheet  prepared  for  the  purpose,  which  has  a  blank  outline 
map  of  the  United  States  at  the  top,  and  space  in  the  lower 
half  for  the  forecasts,  summary,  and  tables. 

The  size  of  the  chalk-plate  map  itself  is  10  by  6^  inches  ; 
the  size  of  the  whole  sheet,  which  includes  also  the  text  and 
tables,  16  by  11  inches.  Weather  maps  prepared  by  the  chalk- 
plate  process  are  now  issued  from  28  of  the  84  stations  which 
publish  daily  maps.  At  the  remaining  stations  the  maps  are 
prepared  by  a  stencil  process,  the  size  of  the  map  being  13|-  by 
22  inches.  The  total  number  of  weather  maps  issued  at  the 
various  stations  during  the  fiscal  year  1897-1898  was 
5,239,300. 

Besides  recording  the  usual  meteorological  data,  and  publish- 


TEMPERATURE.  51 

ing  weather  maps  and  forecasts,  the  various  stations  of  the 
Weather  Bureau  serve  as  distributing  centers  for  cold  wave, 
frost,  flood,  and  storm  warnings.  These  warnings  are  promptly 
sent  out  by  telegraph,  telephone,  and  mail.  Besides  these 
usual  methods  of  distributing  forecasts,  other  means  have  also 
been  adopted.  In  some  places  factory  whistles  are  employed 
to  inform  those  within  hearing  as  to  the  coming  weather  ;  rail- 
way trains  are  provided  with  flags,  whose  various  colors  an- 
nounce to  those  who  are  near  the  train  fair  or  stormy  weather, 
rising  or  falling  temperature  ;  and  at  numerous  so-called  "  dis- 
play stations,"  scattered  all  over  the  country,  the  forecasts  are 
widely  disseminated  by  means  of  flags. 


CHAPTER    V. 

TEMPERATURE. 

A.  Lines  of  Equal  Temperature.  —  Temperature  is  the  most 
important  of  all  tha^jffifiaAer  elements.  It  is  therefore  with 
a  study  of  the  distribution  of  temperature  over  the  United 
States,  and  of  the  manner  of  representing  that  distribution, 
that  we  begin  our  exercises  in  map  drawing.  In  carrying  out 
the  work  we  shall  proceed  in  a  way  similar  to  that  adopted  by 
the  officials  of  the  Weather  Bureau  in  Washington  and  at  the 
other  map-publishing  stations  over  the  country. 

Enter  on  a  blank  weather  map  the  temperature  readings  found 
in  the  first  column  of  the  table  in  Chapter  VIII.  These  read- 
ings are  given  in  degrees  of  the  ordinary  Fahrenheit  scale  [those 
which  are  preceded  by  the  minus  sign  (  — )  being  below  zero], 
and  were  made  at  the  same  time  ( 7  A.M.,  "  Eastern  Standard 
Time  ")  all  over  the  United  States.  Make  your  figures  small 
but  distinct,  and  place  them  close  to  the  different  stations  to 
which  they  belong.  This  is  done  every  morning  at  the  Weather 
Bureau  in  Washington,  when  the  telegraphic  reports  of  weather 


52  CONSTRUCTION    OF    WEATHER    MAPS. 

conditions  come  in  from  all  over  the  country.  When  all  the 
temperature  readings  have  been  entered  on  the  outline  map, 
you  have  before  you  a  view  of  the  actual  temperature  distribu- 
tion over  the  United  States  at  7  A.M.,  on  the  first  day  of  the 
series.  Describe  the  distribution  of  temperature  in  general 
terms,  comparing  and  contrasting  the  different  sections  of  the 
country  in  respect  to  their  temperature  conditions.  Where  are 
the  lowest  temperatures  ?  Where  are  the  highest  ?  What  was 
the  lowest  thermometer  reading  recorded  anywhere  on  the 
morning  of  this  day?  At  what  station  was  this  reading  made? 
What  was  the  highest  temperature  recorded?  And  at  what 
station  was  this  reading  made? 

Notice  that  the  warmest  districts  on  the  map  are  in  Florida, 
along  the  Gulf  Coast,  and  along  the  coast  of  California.  The  marked 
contrasts  in  temperature  between  the  Northwest  and  the  Pacific  and 
Gulf  Coasts  at  once  suggest  a  reason  why  Florida  and  Southern 
California  are  favorite  winter  resorts.  To  these  favored  districts 
great  numbers  of  people  who  wish  to  escape  the  severe  cold  of 
winter  in  the  Northern  States  travel  every  year,  and  here  they  enjoy 
mild  temperature  and  prevailingly  sunny  weather.  To  the  cold 
Northwest,  on  the  other  hand,  far  from  the  warm  waters  of  the 
Pacific,  where  the  days  are  short  and  the  sun  stands  low  in  the  sky, 
no  seekers  after  health  travel.  This  annual  winter  migration  from 
the  cities  of  the  North  to  Florida  and  Southern  California  has  led  to 
the  building  of  great  hotels  in  favored  locations  in  these  States,  and 
during  the  winter  and  spring  fast  express  trains,  splendidly  equipped, 
are  run  from  north  to  south  and.  from  south  to  north  along  the  Atlan- 
tic Coast  to  accommodate  the  great  numbers  of  travelers  between 
New  York,  Philadelphia,  Boston,  Chicago,  and  other  large  northern 
cities,  and  the  Florida  winter  resorts.  Southern  California  also  is 
rapidly  developing  as  a  winter  resort,  and  rivals  the  far-famed 
Eiviera  of  Southern  Europe  as  a  mild  and  sunny  retreat  from  the 
severe  climates  of  the  more  northern  latitudes.  The  control  which 
meteorological  conditions  exercise  over  travel  and  over  habitability  is 
thus  clearly  shown.  Florida  and  Southern  California  are  also  regions 
in  which,  owing  to  the  mildness  of  their  winter  climates,  certain 
fruits,  such  as  oranges  andlemons,  which  are  not  found  elsewhere 


TEMPERATURE.  53 

in  the  country,  can  be  grown  out  of  doors,  and  these  are  shipped  to 
all  parts  of  the  United  States.  i 

Let  us  take  another  step  in  order  to  emphasize  more  clearly' 
the  distribution  of  temperature  over  the  United  States  on  the  first 
day  of  our  series.  Draw  a  line  which  shall  separate  all  places  hav- 
ing a  temperature  above  30°  from  those  having  temperatures  below 
30°,  30°  being  nearly  the  freezing  point  and,  therefore,  a  critical 
temperature.  Evidently  this  will  help  us  to  make  our  descrip- 
tion of  the  temperature  distribution  more  detailed.  If  this  line 
is  to  separate  places  having  temperatures  above  30°  from  those 
having  temperatures  below  30°,  it  must  evidently  pass  through 
all  places  whose  temperature  is  exactly  30°.  Examine  the  ther- 
mometer readings  entered  on  your  map  to  see  whether  there  are 
any  which  indicate  exactly  30°.  You  will  find  this  reading 
at  Norfolk,  Va.,  Wilmington,  S.  C.,  Atlanta,  Ga.,  Chattanooga, 
Tenn.,  Ft.  Smith,  Ark.,, and  Portland,  Ore.  Through  all  these 
stations  the  line  of  30°  must  be  drawn.  Begin  the  line  on  the 
Atlantic  Coast  at  Norfolk,  Va.,  and  draw  it  wherever  you  find  a 
thermometer  reading  of  30°.  It  is  best  to  trace  the  line  faintly 
with  pencil  at  first,  so  that  any  mistakes  can  be  easily  rectified, 
and  it  should  be  drawn  in  smooth  curves,  not  in  angles.  From 
Norfolk  the  line  must  run  southwest  through  Wilmington,  and 
then  westward  through  Atlanta,  passing  just  north  of  Augusta, 
which  has  31°.  From  Atlanta  the  line  goes  northwest  through 
Chattanooga,  and  thence  westward,  curving  south  of  Memphis 
(28°)  and  Little  Rock  (26°),  and  then  northwestward  again 
through  Ft.  Smith. 

In  fixing  the  exact  position  of  the  30°  line  south  of  Memphis 
and  Little  Rock,  the  following  considerations  must  be  our 
guide:  Memphis  has  28°;  Vicksburg  has  35°.  Neither  of  these 
stations  has  30°.  Suppose,  however,  that  you  had  started 
from  Memphis,  with  a  thermometer,  and  had  traveled  very 
rapidly  to  Vicksburg.  The  thermometer  reading  at  starting  in 
Memphis  would  have  been  28°,  and  at  the  end  of  your  journey 


54  CONSTRUCTION    OF    WEATHER    MAPS. 

in  Vicksburg  it  would  have  been  35°,  presuming  that  no 
change  in  temperature  at  either  station  took  place  during 
the  journey.  Evidently  the  mercury  rose  during  the  jour- 
ney, and  in  rising  from  28°  to  35°  it  must,  somewhere  on 
the  way,  have  stood  at  exactly  30°.  Now  this  place,  where  the 
temperature  was  exactly  30°,  is  the  point  through  which  our 
30°  line  ought  to  pass.  How  are  we  to  determine  its  location? 
Assume,  as  is  always  done  in  such  cases,  that  the  temperature 
increased  at  a  uniform  rate  between  Memphis  and  Vicksburg. 
The  total  rise  was  from  28°  to  35°  =  7°.  In  order  to  find  a 
temperature  7°  higher  than  at  Memphis,  you  had  to  travel  the 
whole  distance  from  Memphis  to  Vicksburg.  Suppose  you  had 
only  wished  to  find  a  temperature  5°  higher.  Then,  assuming 
a  uniform  rate  of  increase  between  the  two  stations,  you  would 
have  had  to  travel  only  fy  of  the  distance,  and  your  thermometer 
at  that  place  would  have  read  28°  +  5°  =  33°.  But  assume  you 
had  wanted  to  find  the  place  where  the  thermometer  stood  at 
30°.  In  this  case  you  would  have  been  obliged  to  go  but  f-  of 
the  total  distance  from  Memphis  to  Vicksburg,  and  at  that  point 
your  thermometer  reading  would  have  been  28°  +  2°  =  300,  which 
is  the  point  we  wish  to  find.  In  this  way,  then,  when  we  do 
not  find  the  exact  temperature  we  are  looking  for  on  the  map, 
we  can  calculate  where  that  temperature  prevails  by  noting 
places  which  have  temperatures  somewhat  higher  and  some- 
what lower,  and  proceeding  as  in  the  case  just  described.  Take 
another  example.  Little  Rock,  Ark.,  has  26°;  Shreveport,  La., 
has  40°.  40°  -  26°  =  14°,  which  is  the  total  difference.  From 
26°  to  30°  is  4°.  Therefore  a  point  T\  or  f  of  the  distance 
from  Little  Rock  to  Shreveport  should  have  a  temperature  of 
26°  +  4°  =  30°,  which  is  the  point  we  wish  to  find,  and  through 
which  our  30°  line  must  pass. 

From  Ft.  Smith  the  line  cannot  go  north  or  northwest  or 
west,  because  the  temperatures  there  are  all  below  30°.  To  the 
south  the  temperatures  are  all  above  30°.  Evidentlv  there  is 


TEMPERATURE.  55 

only  one  direction  in  which  you  can  prolong  the  line,  and  that 
is  to  the  southwest.  Temperatures  of  30°  cannot  be  found 
north  of  El  Paso  (28°),  because  there  the  temperature  distinctly 
falls,  Santa  Fe  having  4°,  Denver,  -  14°,  and  Cheyenne,  -  23°. 
Therefore  temperatures  above  28°  must  be  found  south  of  El 
Paso.  From  Ft.  Smith  you  may,  therefore,  continue  the  30°  line 
southwest  and  west,  passing  close  to  El  Paso,  but  to  the  south 
of  it.  In  determining  the  further  course  of  the  30°  line,  note 
that  Yuma  and  all  the  California  stations  have  temperatures 
above  30°,  while  Winnemucca,  Nev.,  has  13°,  and  Portland,  Ore., 
has  exactly  30°.  From  El  Paso  you  may,  therefore,  continue 
the  line  to  the  northwest,  passing  up  through  Central  California 
parallel  with  the  coast  line,  and  to  the  east  of  all  the  California 
stations  and  of  Roseburg,  Ore.,  and  thence  running  through 
Portland,  Ore.,  ending  just  west  of  Seattle,  Wash.  Notice  that 
the  30°  line  should  be  nearer  to  Sacramento,  Cal.,  with  36°,  than 
to  Red  Bluff  with  44°. 

Thus  you  have  drawn  the  line  which  passes  through  all 
places  that  have  a  temperature  of  30°  on  the  map  under  dis- 
cussion. This  may  be  called  a  line  of  equal  temperature. 
Isotherm,  a  compound  of  two  Greek  words  meaning  equal 
temperature,  is  the  name  given  in  meteorology  to  such  lines 
as  this.  You  have  drawn  the  isotherm  of  30°.  All  parts  of 
the  United  States  north  and  east  of  this  line  are  below  30°, 
while  all  districts  south  and  west  of  it  are  above  30°.  You  see, 
therefore,  how  much  easier  the  drawing  of  this  one  line  has 
made  the  description  of  the  temperature  distribution  over  the 
United  States. 

Carry  this  process  a  step  further  by  drawing  the  line  which 
shall  pass  through  all  places  with  a  temperature  of  40°.  This 
line  begins  at  Jacksonville,  Fla.  (40°),  and  runs  west,  passing 
between  Montgomery,  Ala.  (33°),  and  Pensacola,  Fla.  (46°). 
Thence  it  turns  to  the  northwest,  passing  between  Vicksburg, 
Miss.  (35°),  and  New  Orleans,  La.  (48°),  and  through  Shreve- 


56  CONSTRUCTION    OF    WEATHEK    MAPS. 

port,  La.  (40°).  From  Shreveport  it  turns  to  the  southwest, 
passing  to  the  north  and  west  of  Palestine,  Tex.  (46°),  arid  down 
through  San  Antonio,  Tex.  (40°).  Its  further  exact  location 
cannot  be  determined  in  Mexico,  because  there  are  no  observa- 
tions from  Mexican  stations,  but  the  readings  at  Yuma,  Ariz. 
(41°),  and  at  San  Diego  (42°),  Los  Angeles  (44°),  San  Francisco 
(45°),  Red  Bluff  (44°),  and  Cape  Mendocino  (43°),  all  in  Cali- 
fornia, show  that  the  40°  isotherm  may  be  started  again  just 
north  of  Yuma,  arid  may  be  carried  up  through  California, 
nearly  parallel  with  the  Pacific  Coast,  ending  between  Cape 
Mendocino,  Cal.  (43°),  and  Roseburg,  Ore.  (37°).  You  have 
now  drawn  the  isotherms  of  30°  and  of  40°,  and  in  order  to 
avoid  confusion,  mark  the  ends  of  the  first  line  30°  and  the 
ends  of  the  second  line  40°. 

Isotherms  on  weather  maps  are  drawn  for  every  even  10°  of 
temperature.  They  are  drawn  in  smooth  curves  and  not  in 
angular  sections.  Two  isotherms  cannot  cross  one  another,  for 
if  they  did  you  would  have  tAvo  temperatures,  differing  by  10°, 
at  the  point  of  crossing,  which  is  obviously  impossible.  Com- 
plete the  chart  for  this  day  by  drawing  the  remaining  isotherms, 
i.e.,  those  for  50°,  20°,  10°,  0°,  -  10°,  -  20°,  and  -  30°,  bearing 
in  mind  what  has  been  said  in  regard  to*  the  determination  of 
the  positions  of  isotherms  when  the  exact  temperature  you  are 
seeking  is  not  given  on  the  map. 

The  dotted  lines  in  Fig.  18  show  the  positions  of  the  isotherms 
when  drawn.  Notice  how  clearly  the  temperature  distribution 
now  stands  out,  and  how  simple  the  description  of  that  distribu- 
tion has  become.  Observe  that  the  isotherms,  although  more 
or  less  irregular,  show  a  good  deal  of  uniformity  in  their  gen- 
eral courses,  and  this  uniformity  is  a  great  assistance  in  drawing 
them.  Study  the  distribution  of  temperature  on  this  map,  and 
the  positions  of  the  isotherms,  very  carefully. 

Construct  isothermal  charts  for  the  remaining  days  of  the 
eries.  Use  a  new  blank  map  for  each  day,  and  take  the  tempera- 


TEMPERATURE. 


57 


ture  observations  from  the  table  in  Chapter  VIII.  Proceed  as 
in  the  case  of  the  first  day.  Draw  the  isotherms  for  every  even 
10°  of  temperature,  taking  care  to  study  the  course  of  each  line 


FIG.  18.  — Isotherms.    First  day. 

before  you  begin  to  draw  the  line.  The  charts  when  completed 
form  a  series  in  which  the  temperature  distribution  over  the 
United  States  is  shown  at  successive  intervals  of  24  hours. 

In  order  to  bring  out  the  temperature  distribution  on  the 
maps  more  clearly,  color  (with  colored  pencils  or  water  colors) 
all  that  portion  of  each  map  which  lies  within  the  —  20°  isotherm 
a  dark  blue ;  that  portion  which  is  between  the  0°  isotherm  and 
the  —  20°  isotherm  a  somewhat  lighter  shade  of  blue,  and  those 
districts  which  are  between  0°  and  -f-  30°  a  still  lighter  blue. 
The  portion  of  the  map  above  30°  and  below  40°  may  be  left 
uncolored,  while  the  districts  having  temperatures  over  40°  may 
be  colored  red.  In  the  map  for  the  third  day  the  district  which 
has  temperatures  below  —50°  should  be  colored  darker  blue 
than  any  shade  used  on  the  other  maps,  or  black,  in  order  to 


58 


CONSTRUCTION   OF   WEATHER   MAPS. 


.  19. —  Temperature.    First  Day 


FIG.  20.  — Temperature.    Second  Day. 

emphasize  the  extremely  low  temperatures  there  found.     Figs. 
19-24,  on  which  the  isotherms  are  shown,  also  illustrate  the 


TEMPERATURE. 


59 


FIG.  21.  — Temperature.    Third  Day. 


FIG.  22.  — Temperature.    Fourth  Day. 

appearance  of  these  maps  when  the  different  temperature  areas 
are  colored,  as  has  been  suggested. 


60 


CONSTRUCTION    OF    WEATHER    MAPS. 


FIG.  23.  —  Temperature.    Fifth  Day 


FIG.  24.  —  Temperature.    Sixth  Day. 

Study  the  maps  individually  at  first.     Describe  the  temper- 
ature distribution  on  each  map.     Ask  yourself  the  following 


TEMPERATURE.  61 

If* 

questions  in  each  case  :  Where  is  it  coldest?  Where 
warmest  ?  What  is  the  lowest  temperature  on  the  map  ? 
What  is  the  highest?  At  what  stations  were  these  readings 
made? 

Then  compare  the  successive  maps  and  answer  these  ques- 
tions :  What  changes  have  taken  place  in  the  intervening 
L  24  hours  ?  In  what  districts  has  the  temperature  risen  ? 
What  is  the  greatest  rise  that  has  occurred?  Where?  In 
what  districts  has  the  temperature  fallen?  What  was  the 
greatest  fall  in  temperature  and  where  did  it  occur?  Has  the 
temperature  remained  nearly  stationary  in  any  districts?  In 
which?  You  will  find  it  a  help  in  answering  such  questions 
to  make  out  a  table  of  all  the  stations,  and  to  indicate  in 
columns,  after  the  names  of  the  stations,  the  number  of  degrees 
of  rise  or  fall  in  temperature  at  each  place  during  the  24-hour 
interval  between  the  successive  maps.  When  the  tempera- 
ture is  higher  at  any  station  than  it  was  on  the  preceding 
day,  note  this  by  writing  a  plus  sign  (+)  before  the  number 
of  degrees  of  rise  in  temperature.  When  the  temperature  has 
fallen,  put  a  minus  sign  (— )  before  the  number  of  degrees  of 
fall.  Thus,  New  Orleans,  La.,  had  a  temperature  of  48°  on 
the  first  day.  On  the  second  it  had  33°.  Therefore  the  change 
at  New  Orleans  was  —  15°  in  the  24  hours.  At  Key  West, 
Fla.,  the  change  was  +11°  in  the  same  time. 

Write  a  brief  account  of  the  temperature  distribution  on 
each  day  of  the  series,  and  of  the  changes  which  took  place 
between  that  day  and  the  one  preceding,  naming  the  districts 
and  States  over  which  the  most  marked  falls  and  rises  in  tem- 
perature occurred,  with  some  indication  of  the  amount  of  these 
changes.  Note  especially  the  changes  in  position,  and  the 
extent,  of -the  districts  with  temperatures  below  —  20°;  between 
0°  and  -20°,  and  between  30°  and  0°.  Write  out  a  clear,  con- 
cise statement  of  the  temperature  distribution  and  changes 
shown  on  the  whole  set  of  six  maps. 


62  CONSTRUCTION   OF   WEATHER   MAPS. 

Cold  Waves, The  series  of  charts  for  these  six  days  fur- 
nishes an  excellent  illustration  of  a  severe  cold  wave. 

A  cold  wave,  as  the  term  is  now  used  by  the  Weather  Bureau, 
means,  during  December,  January,  and  February,  a  fall  in  tempera- 
ture of  from  20°  to  16°  in  24  hours,  with  a  resulting  reduction  of 
temperature  to  between  0°  and  32°,  and,  during  the  months  from 
March  to  November  inclusive,  a  fall  of  from  20°  to  16°  in  24  hours, 
with  a  reduction  of  temperature  to  from  16°  to  36°.  During 
December,  January,  and  February  a  cold  wave  means  the  following 
falls  and  reductions  of  temperature.  Over  the  Northwestern  States, 
from  western  Wisconsin  to  Montana,  including  Wyoming,  Nebraska, 
and  western  Iowa,  and  over  northeastern  New  York  and  northern 
New  Hampshire,  northern  Vermont  and  northern  Maine,  a  fall  of 
20°  or  more  to  zero  or  below;  over  southern  New  England  and 
adjoining  districts,  the  Lake  region,  the  central  valleys  and  west  to 
Colorado,  including  northern  New  Mexico  and  northwestern  Texas, 
a  fall  of  20°  or  more  to  10°  or  below  ;  over  southern  New  Jersey, 
Delaware,  eastern  Maryland,  Virginia,  western  North  Carolina, 
northwestern  South  Carolina,  northern  Georgia,  northern  Alabama, 
northern  Mississippi,  Tennessee,  southern  Kentucky,  Arkansas, 
Oklahoma,  and  southern  New  Mexico,  a  fall  of  20°  or  more  to  20° 
or  below ;  over  eastern  North  Carolina,  central  South  Carolina, 
central  Georgia,  central  Alabama,  central  Mississippi,  central  and 
northern  Louisiana  and  central  and  interior  Texas,  a  fall  of  18°  or 
more  to  25°  or  below  ;  along  the  Gulf  coasts  of  Texas,  Louisiana, 
Mississippi,  and  Alabama,  over  all  of  Florida,  and  over  the  coasts 
of  Georgia  and  South  Carolina,  a  fall  of  16°  or  more  to  32°  or  below: 
From  March  to  November  inclusive  a  cold  wave  means  falls  of  tem- 
perature of  the  same  amounts  over  the  same  districts,  with  result- 
ing temperatures  of  16°,  24°,  28°,  32°,  and  36°  respectively. 

Notice  that  the  region  from  which  the  greatest  cold  came  in 
this  cold  wave  is  Canada.  In  that  northern  country,  with 
its  short  days  and  little  sunshine,  and  its  long,  cold  nights, 
everything  is  favorable  to  the  production  of  very  low  tem- 
peratures. 

Cold  waves  occur  only  in  winter.  In  the  summer  cool 
spells,  with  similar  characteristics,  may  be  called  cool  waves. 


TEMPERATURE.  63 

Cold- Wave  Forecasts.  —  A  severe  cold  wave  in  winter  does 
much  damage  to  fruit  and  crops  growing  out  of  doors  in  our 
Southern  States,  and  to  perishable  food  products  in  cars,  on  the 
way  from  the  South  to  supply  the  great  cities  of  the  North. 
Therefore  it  is  important  that  warnings  should  be  issued  giv- 
ing early  information  of  the  coming  cold,  so  that  farmers  and 
fruit  growers  and  shippers  may  take  every  precaution  to  protect 
their  crops  and  produce.  Our  Weather  Bureau  takes  special 
pains  to  study  the  movements  of  cold  waves  and  to  make  fore- 
casts of  them,  and  so  well  are  the  warnings  distributed  over  the 
country  that  the  fruit  growers  and  the  transportation  companies, 
and  the  dealers  in  farm  produce,  are  able  every  winter  to  save 
thousands  of  dollars'  worth  of  fruit  and  vegetables  which 
would  otherwise  be  lost.  Cold-wave  warnings  are  heeded 
by  many  persons  besides  those  who  are  directly  interested  in 
fruits  and  farm  products.  The  ranchmen  in  the  West,  with 
thousands  of  cattle  under  their  charge  ;  the  trainmen  in 
charge  of  cattle  trains  ;  the  engineers  of  large  buildings,  such 
as  hotels,  stores,  and  office  buildings,  who  must  have  their 
fires  hotter  in  cold  weather,  —  these  and  many  more  watch, 
and  are  governed  by,  the  cold-wave  forecasts  of  our  Weather 
Bureau. 

Mean  Annual  and  Mean  Monthly  Isothermal  Charts.  —  We  have 
thus  far  considered  isothermal  charts  for  the  United  States  only, 
based  on  the  temperature  observations  made  at  a  single  moment  of 
time.  It  is,  of  course,  possible  to  draw  isothermal  charts,  the  data 
for  which  are  not  the  temperatures  at  a  given  moment,  but  are  the 
mean  or  average  temperatures  for  a  month  or  a  year.  Such  charts 
have  been  constructed  for  other  countries  besides  our  own,  as  well 
as  for  the  whole  world.  An  isothermal  chart  based  on  the  mean 
annual  temperatures  is  known  as  a  mean  annual  isothermal  chart. 
These  charts  show  at  once  the  average  distribution  of  temperature 
for  the  month  or  for  the  year,  just  as  the  ones  we  have  drawn  show 
the  distribution  of  temperature  over  the  United  States  at  a  single 
moment. 


64  CONSTRUCTION    OF    WEATHER    MAPS. 

B.  Direction  and  Rate  of  Temperature  Decrease.  Temperature 
Gradient.— Take  your  isothermal  map  for  the  first  day  and  imag- 
ine yourself  at  Kansas  City,  Mo.  In  what  direction  must  you 
go  from  Kansas  City  in  order  to  enter  most  rapidly  into  colder 
weather?  In  what  direction  must  you  go  from  Kansas  City  in 
order  to  enter  most  rapidly  into  warmer  weather?  Take  the 
case  of  Salt  Lake  City.  In  what  direction  must  you  go  from 
that  station  in  order  to  enter  most  rapidly  into  colder  weather? 
Into  warmer  weather  ?  What  are  the  corresponding  directions 
in  the  case  of  Spokane,  Wash/?  Of  Bismarck,  N.  Dak.?  Of 
Buffalo,  N.  Y.  ?  Of  Montreal,  Que.  ?  Of  Portland,  Me.  ?  Of 
Sacramento,  Cal.  ? 

Draw  a  line  from  Kansas  City  to  the  nearest  point  at  which 
there  is  a  temperature  10°  lower  than  at  Kansas  City.  Evi- 
dently this  point  is  on  the  isotherm  of  0°,  and  will  be  found  if 
a  line  be  drawn  from  Kansas  City  towards,  and  at  right  angles 
to,  the  isotherm  of  0°.  Continue  the  line  beyond  the  0°  iso- 
therm in  the  direction  of  still  lower  temperatures,  i.e.,  to  the 
isotherms  of  —  10°,  —  20°,  and  —  30°.  Beyond  the  isotherm  of 
-  30°  the  line  must  stop.  Draw  similar  lines  from  Seattle, 
Wash.;  Salt  Lake  City,  Utah;  Denver,  Col.;  St.  Paul,  Minn.; 
Cleveland,  O.;  and  New  York,  N.  Y.  Prolong  these  lines  all 
across  the  map,  so  that  they  will  extend  from  the  regions  of 
highest  temperature  to  those  of  the  lowest.  A  number  of  inter- 
mediate lines  may  also  be  added.  Note  that  the  various  direc- 
tions followed  by  these  lines  are  square  to,  or  at  right  angles 
to,  the  successive  isotherms,  and  that  although  the  lines  all  run 
from  higher  to  lower  temperatures,  they  do  not  all  trend  in  the 
same  direction.  These  lines  may  be  called  lines  of  decrease  of 
temperature.  Fig.  25  shows  a  few  of  these  lines  of  decrease  of 
temperature  drawn  for  the  first  day. 

Draw  similar  lines  on  the  other  isothermal  charts,  for  the 
same  stations.  Are  the  directions  of  temperature  decrease  the 
same  on  these  charts  as  on  the  chart  for  the  first  day,  for  Kansas 


TEMPERATURE. 


65 


City,  Seattle,  Salt  Lake  City,  Denver,  St.  Paul,  Cleveland,  New 
York  ?  Draw  lines  of  decrease  of  temperature  from  the  follow- 
ing additional  stations  :  Key  West,  Fla.  ;  New  Orleans,  La.  ; 
.;  El  Paso,  Tex.;  San  Diego,  Gal.;  Hattcraa,  N.  O. 


Compare  the  directions  of  these  lines  on  the  different  days. 
How  do  they  change  from  one  day  to  the  next  ? 

Next  select  some  line  of  decrease  of  temperature  on  the  map 
for  the  first  day  which  begins  in  Texas,  and  follow  it  northward. 


FIG.  25.  —  Temperature  Gradients.    First  Day. 

Where,  along  this  line,  is  the  decrease  of  temperature  most 
rapid  ?  Evidently  this  must  be  where  the  isotherms  are  closest 
together,  because  every  isotherm  that  is  crossed  means  a  change 
of  temperature  of  10°,  and  the  more  isotherms  there  are  in  a 
given  distance,  the  more  rapidly  the  temperature  is  changing. 
Where  the  isotherms  are  closest  together,  a  given  decrease  of 
temperature  is  passed  over  in  the  least  distance,  or,  conversely, 
a  greater  decrease  of  temperature  is  experienced  in  a  given  dis- 
tance. Study  this  question  of  rapidity  or  slowness  of  tempera- 
ture decrease  on  the  whole  series  of  charts.  On  which  of  the 


66  CONSTRUCTION    OF    WEATHER    MAPS. 

charts,  and  where,  do  you  find  the  most  rapid  decrease  ?  The 
slowest  decrease  ?  Is  there  any  regularity  in  these  rates  of  tem- 
perature decrease  either  on  one  map  or  in  the  whole  series  of 

maps  ? 

The  term  temperature  gradient  is  used  by  meteorologists  to 
describe  the  direction  and  rate  of  temperature  decrease  which  we 
have  been  studying. 

If  we  are  to  compare  these  rates  of  temperature  change,  we 
must  have  some  definite  scale  of  measurement.  Thus,  for 
example,  in  speaking  of  the  wind  velocity  we  say  the  velocity 
of  the  wind  is  so  many  miles  per  hour  ;  in  describing  the  grade 
of  a  railroad  we  say  it  is  so  many  feet  in  a  mile.  In  dealing 
with  these  temperature  changes,  we  adopt  a  similar  scheme. 
We  say  :  The  rate  of  temperature  decrease  is  so  many  degrees 
Fahrenheit  in  a  distance  of  one  latitude  degree  (about  70  miles). 
In  order  to  make  our  measurements,  we  use  a  scale  of  latitude 
degrees,  just  as,  in  calculating  railroad  grades,  we  must  have 
a  way  to  measure  the  miles  of  track  in  which  the  ascent  or 
descent  of  the  roadbed  is  so  many  feet.  Take  a  strip  of  paper 
6  inches  long,  with  a  straight  edge,  and  lay  this  edge  north 
and  south  at  the  middle  of  the  weather  map,  along  a  longitudi- 
nal or  meridian  line.  Mark  off  on  the  strip  of  paper  the  points 
where  any  two  latitude  lines  cross  the  meridian  line.  These 
latitude  lines  are  five  (latitude)  degrees  apart.  Therefore 
divide  the  space  between  them  on  your  paper  into  five  divi- 
sions, and  each  of  these  will  measure  just  one  latitude  degree. 
Continue  making  divisions  of  the  same  size  until  you  have  ten 
altogether  on  the  strip  of  paper.  Select,  on  any  weather  map, 
some  station  lying  between  two  isotherms  at  which  you  wish  to 
measure  the  rate  of  temperature  decrease.  Take,  for  instance, 
Buffalo,  N.  Y.,  on  the  first  day.  What  you  want  to  find 
is  this  :  What  is  the  rate  of  temperature  decrease,  or  the  tem- 
perature gradient,  at  Buffalo?  Lay  your  paper  scale  of  lati- 
tude degrees  through  Buffalo,  from  the  isotherm  of  10°  to  the 


TEMPERATURE.  67 

isotherm  of  0°,  and  as  nearly  as  possible  at  right  angles  to  the 
isotherms.1  Count  the  number  of  latitude  degrees  on  your  scale 
between  the  isotherms  of  10°  and  0°,  on  a  line  running  through 
Buffalo.  There  are,  roughly,  about  two  degrees  of  latitude  in 
this  distance.  That  is,  in  the  district  in  which  Buffalo  lies,  the 
temperature  is  changing  at  the  rate  of  10°  Fahrenheit  (between 
isotherms  10°  and  0°)  in  two  latitude  degrees.  As  our  standard 
of  measurement  is  the  amount  of  change  of  temperature  in  one 
latitude  degree,  we  divide  the  10  (the  number  of  degrees  of 
temperature)  by  the  2  (the  number  of  degrees  of  latitude),  which 
gives  us  5  as  the  rate  of  decrease  of  temperature  per  latitude 
degree  at  Buffalo,  N.  Y.,  at  7  A.M.,  on  the  first  day  of  the  series. 
The  temperature  gradient  at  Buffalo  is  therefore  5.  The  rule 
may  be  stated  as  follows  :  Select  the  station  for  which  you  wish 
to  know  the  rate  of  temperature  decrease  or  temperature  gra- 
dient. Lay  a  scale  of  latitude  degrees  through  the  station,  and 
as  nearly  as  possible  at  right  angles  to  the  adjacent  isotherms. 
If  the  station  is  exactly  on  an  isotherm  then  measure  the  dis- 
tance from  the  station  to  the  nearest  isotherm  indicating  a 
temperature  10°  lower.  The  scale  must,  however,  be  laid  per- 
pendicularly to  the  isotherm,  as  before.  Divide  the  number  of 
degrees  of  difference  of  temperature  between  the  isotherms 
(always  10°)  by  the  distance  (in  latitude  degrees)  between  the 
isotherms,  and  the  quotient  is  the  rate  of  temperature  decrease 
per  latitude  degree.  Or,  to  formulate  the  operation  : 

T 

*-*, 

in  which  R  =  rate  ;  T  ==  temperature  difference  between  iso- 
therms (always  10°),  and  D  =  distance  between  isotherms  in 
latitude  degrees.  Thus,  a  distance  of  10  latitude  degrees 
gives  a  rate  of  1  ;  a  distance  of  5  gives  a  rate  of  2  ;  a  dis- 

1  Unless  the  isotherms  are  exactly  parallel,  the  scale  cannot  be  at  right  angles 
to  both  of  them.  It  should,  however,  be  placed  as  nearly  as  possible  in  that 
position. 


68  CONSTRUCTION    OF    WEATHEK   MAPS. 

tance  of  2  gives  a  rate  of  5  ;  a  distance  of  4  gives  a  rate  of 

2.5,  etc. 

Determine  the  rates  of  temperature  decrease  in  the  following 

cases : — 

A.  For  a  considerable  number  of  stations  in  different  parts  of 
the  same  map,  as  for  each  of  the  six  days  of  the  series. 

And,  using  the  school  file  of  weather  maps, 

B.  For  one  station  during  a  winter  month   and  during  a 
summer  month,  measuring  the  rate  on  each  map  throughout  the 
month  and  obtaining  an  average  rate  for  the  month. 

O.  For  a  station  on  the  Pacific  Coast,  and  one  on  the  Atlantic 
Coast  during  the  same  months. 

D.  For  a  station  on  the  Gulf  of  Mexico,  or  in  Florida,  and 
one  in  the  Northwest  during  a  winter  month. 

E.  For  a  station  in  the  central  United  States,  and  one  on  the 
Pacific  Coast,  the  Gulf  Coast,  and  the  Atlantic  Coast,  respec- 
tively, during  different  months  of  the  winter  and  summer. 

The  determination  of  the  rates  of  temperature  decrease  under 
these  different  conditions  over  the  United  States  prepares  us 
for  an  appreciation  of  the  larger  facts,  of  a  similar  kind,  to  be 
found  on  the  mean  annual  and  mean  monthly  isothermal  charts 
of  various  countries,  and  also  of  the  whole  world. 

Temperature  Gradients  on  Isothermal  Charts  of  the  Globe.  —  The 
mean  annual  isothermal  charts  of  the  globe  (see  page  63)  bring 
out  some  very  marked  contrasts  in  rates  of  temperature  decrease. 
Thus,  along  the  eastern  side  of  the  North  American  continent  the 
isotherms  are  crowded  close  together,  while  on  the  western  coast  of 
Europe  they  are  spread  far  apart.  Between  southern  Florida  and 
Maine  there  is  the  same  change  in  mean  annual  temperature  as  is 
found  between  the  Atlantic  coast  of  the  Sahara  and  central  England. 
The  latter  is  a  considerably  longer  distance,  and  this  means  that 
the  decrease  of  temperature  is  much  slower  on  the  European  Atlan- 
tic coast  than  on  the  North  American  Atlantic  coast.  In  fact,  the 
rate  of  temperature  decrease  with  latitude  in  the  latter  case  is  the 
most  rapid  anywhere  in  the  world,  in  the  same  distance.  These 


TEMPERATURE.  69 

great  contrasts  in  temperature  which  occur  within  short  distances 
along  the  eastern  coast  of  North  America  have  had  great  influence 
upon  the  development  of  this  region,  an  hu&  been  puinled  "5u£~T)y 
Weeikefr^nr^eMMire^^  The  products  of  the 

tropics  and  of  the  Arctic  are  here  brought  very  near  together ;  and 
at  the  same  time  intercommunication  between  these  two  regions  of 
widely  differing  climates  is  very  easy.  Labrador  is  climatically  an 
Arctic  land,  and  man  is  there  forced  to  seek  his  food  chiefly  in  the 
sea,  for  nature  supplies  him  with  little  on  shore,  while  southern 
Florida  is  quite  tropical  in  its  temperature  conditions  and  in  the 
abundance  of  its  vegetation.  Between  the  Pacific  coasts  of  Asia 
and  of  North  America  there  is  a  similar  but  less  pronounced  contrast, 
the  isotherms  being  crowded  together  on  the  eastern  coast  of  China 
and  Siberia,  and  being  spread  apart  as  they  cross  the  Pacific  Ocean 
and  reach  our  Pacific;  Coast. 

In  general,  we  naturally  expect  to  find  that  the  temperature 
decreases  as  one  goes  poleward  from  the  equator ;  from  lower  lati- 
tudes, Avhere  the  sun  is  always  high  in  the  heavens,  to  higher 
latitudes,  where  it  is  near  the  horizon,  and  its  warming  effect  is  less. 
But  there  are  some  curious  exceptions  to  this  general  rule.  The 
lowest  temperatures  on  the  January  isothermal  chart  (  —  60°)  are 
found  in  northeastern  Siberia,  and  not,  so  far  as  our  observations 
go,  near  -the  North  Pole.  If  you  find  yourself  at  this  •'•'  cold  pole," 
as  it  is  called,  in  Siberia  in  January,  you  can  reach  higher  tempera- 
tures by  traveling  north,  south,  east,  or  west.  In  other  words,  here 
is  a  case  of  increase  of  temperature  in  a  northerly  direction,  as  well 
as  east,  south,  and  west.  Again,  there  is  a  district  of  high  tempera- 
ture (90°)  over  southern  Asia  in  July,  from  which  you  can  travel 
south  towards  the  equator  and  yet  reach  lower  temperatures. 

In  our  winter  months  the  contrasts  of  temperature  in  the  United 
States  are,  as  a  rule,  violent,  there  being  great  differences  between 
the  cold  of  the  Northwest  and  the  mild  air  of  Florida  and  the  Gulf 
States.  In  the  summer,  on  the  other  hand,  the  distribution  of 
temperature  is  relatively  equable,  the  isotherms  being,  as  a  rule,  far 
apart.  In  summer,  therefore,  we  approach  the  conditions  charac- 
teristic of  the  Torrid  Zone.  These  are  uniformly  high  temperatures 
over  large  areas.  The  same  thing,  on  a  larger  scale,  is  seen  over 
the  whole  Northern  Hemisphere.  During  our  winter  months  the 
isotherms  are  a  good  deal  closer  together  than  they  are  during  the 


70  CONSTRUCTION    OF    WEATHER    MAPS. 

summer,  or,  in  more  technical  language,  the  temperature  gradient 
between  the  equator  and  the  North  Pole  is  steeper  in  winter  than 
in  summer. 


CHAPTER    VI. 

WINDS. 

THE  observational  work  already  clone,  whether  non-instru- 
mental or  instrumental,  has  shown  that  there  is  a  close  relation 
between  the  direction  of  the  wind  at  any  station  and  the  temper- 
ature at  that  station.  Our  second  step  in  weather-map  drawing 
is  concerned  with  the  winds  011  the  same  series  of  maps  which 
we  have  thus  far  been  studying  from  the  point  of  view  of 
temperature  alone. 

In  the  second  column  of  the  table  in  Chapter  VIII  are  given 
the  wind  directions  and  the  wind  velocities  (in  miles  per  hour) 
recorded  at  the  Weather  Bureau  stations  at  7  A.M.,  on  the 
first  day  of  the  series.  Enter  on  a  blank  Aveather  map,  at  each 
station  for  Avhich  a  wind  observation  is  given  in  the  table,  a 
small  arrow  flying  with  the  wind,  i.e.,  pointing  in  the  direction 
towards  which  the  wind  is  blowing.  Make  the  lengths  of  the 
wind  arrows  roughly  proportionate  to  the  velocity  of  the  wind, 
the  winds  of  higher  velocities  being  distinguished  by  longer 
arrows,  and  those  of  lower  velocities  by  shorter  arrows.  The 
letters  Lt.  (=  light)  in  the  table  denote  wind  velocities  of  5 
miles,  or  less,  per  hour. 

When  you  have  finished  drawing  these  arrows,  you  will  have 
before  you  a  picture  of  the  wind  directions  and  velocities  all 
over  the  United  States  at  the  time  of  the  morning  observation 
on  this  day.  (See  solid  arrows  in  Fig-  26.) 

The  wind  arrows  on  your  map  show  the  wind  directions  at 
only  a  few  scattered  points  as  compared  with  the  vast  extent 
of  the  United  States.  We  must  remember  that  the  whole 
lower  portion  of  the  atmosphere  is  moving,  and  not  merely  the 


WINDS. 


71 


winds  at  these  scattered  stations.  It  will  help  you  to  get  a 
clearer  picture  of  this  actual  movement  of  the  atmosphere  as  a 
whole,  if  you  draw  some  additional  wind  arrows  between  the 
stations  of  observation,  but  in  sympathy  with  the  observed  wind 
directions  given  in  the  table  and  already  entered  on  your  map. 
These  new  arrows  may  be  drawn  in  broken  lines,  and  may  be 
curved  to  accord  in  direction  with  the  surrounding  wind  arrows. 
Heavier  or  longer  lines  may  be  used  to  indicate  faster  winds. 
(See  broken  arrows,  Fig.  26.) 


FIG.  26.  — Winds.    First  Day. 

It  is  clear  that  the  general  winds  must  move  in  broad  sweep- 
ing paths,  changing  their  directions  gradually,  rather  than  in 
narrow  belts,  with  sudden  changes  in  direction.  Therefore  long 
curving  arrows  give  a  better  picture  of  the  actual  drift  of 
the  atmospheric  currents  than  do  short,  straight,  disconnected 
arrows. 

Study  the  winds  on  this  chart  with  care.  Describe  the  condi- 
tions of  wind  distribution  in  a  general  way.  Can  you  discover 
any  apparent  relation  between  the  different  wind  directions 


72 


CONSTRUCTION    OF    WEATHER    MAPS. 


in  any  part  of  the  map  ?  Is  there  any  system  whatever  in  the 
winds?  Write  out  a  brief  and  concise  description  of  the 
results  of  the  study  of  this  map. 

Enter  on  five  other  blank  maps  the  wind  directions  given  in 
the  table  in  Chapter  VIII  for  the  other  five  days  of  the  series, 
making,  as  before,  the  lengths  of  the  arrows  roughly  propor- 
tionate to  the  velocity  of  the  wind,  and  adding  extra  broken 
arrows  as  suggested.  (See  Figs.  27-31.) 

A.  Study  the  whole  series  of  six  maps.     Describe  the  wind 
conditions  on  each  map  by  itself,  noting  carefully  any  system 
in  the  wind  circulation  that  you  may  discover.     Examine  the 
wind   velocities    also.     Are   there   any  districts   in   which   the 
velocities  are  especially  high?     Have  these  velocities  any  re- 
lation to  whatever  wind  systems  you  may  have  discovered  ?     If 
so,  include  in  your  description  of  these  systems  some  considera- 
tion of  the  wind  velocities  as  well  as  of  the  wind  directions. 

B.  Compare  each  map  of  the  series  with  the  map  preceding 
it.     Note  what  changes  in  direction  and  velocity  have  taken 


FIG.  27.  —  Winds.    Second  Day. 


WINDS. 


73 


FIG.  28.  — Winds.    Third  Day. 


FIG.  29.  — "Winds.    Fourth  Day. 


place  at  individual  stations.     Group  these  changes  as  far  as  pos- 
sible by  the  districts  over  which  similar  changes  have  occurred. 


74 


CONSTRUCTION    OF    WEATHER    MAPS. 


FIG.  30.  — Winds.    Fifth  Day. 


95°  Q°  5°  80°  75°  70°" 


FIG.  31.  — Winds.     Sixth  Day. 


Compare  the  wind  systems  on  each  map  with  those   on  the 
map  for  the  preceding  day.     Has  there  been  any   alteration 


WINDS.  TO 

in  the  position  or  relation  of  these  systems  ?  Write  for  each 
day  an  account  of  the  conditions  011  that  map,  and  of  the  changes 
that  have  taken  place  in  the  preceding  24-hour  interval. 

C.  Write  out  a  short  connected  account  of  the  wind  coiidi- 
ktioiis  and  changes  illustrated  on  the  whole  set  of  six  maps. 

In  the  last  chapter  we  studied  the  progression  of  the 

i,ve  of  low  temperatures  in  an  easterly  direction  -  across  the 
United  States.  Notice  now  the  relation  of  the  winds  011  the 
successive  maps  of  our  series  to  the  movement  of  the  cold  wave. 
Place  your  Avind  charts  and  isothermal  charts  for  the  six  days 
side  by  side,  and  study  them  together.  The  temperature  dis- 
tribution 011  the  second  day  differs  from  that  on  the  first.  What 
are  the  chief  differences?  Examine  the  wind  charts  for  these 
two  days.  Do  you  detect  any  differences  in  the  wind  directions 
or  systems  on  these  days  ?  Do  these  differences  help  to  explain 
some  of  the  changes  in  temperature  ? 

Compare  the  temperature  distribution  011  the  second  day  with 
that  on  the  third.     What  are  the  most  marked  changes  in  the 
distribution  ?     What  changes  in  the  winds  011  the  corresponding 
wind  maps  seem  to  offer  an  explanation  of  these  variations? 
X     Proceed  similarly  with  each  map  of  the  series.     Formulate 
injvmting,  the  general  relation  between  winds  and  cold  wave, 
discovered  through  your  study  of  these  charts. 

Cold  Waves  in  Other  Countries.  —  Cold  waves  in  the  United  States 
come,  as  has  been  seen,  from  the  northwest,  that  being  the  region 
of  greatest  winter  cold.  In  Europe,  cold  waves  come  from  the 
northeast.  This  is  because  northwest  of  Europe  there  is  a  large 
body  of  warm  water  supplied  by  the  Gulf  Stream  drift,  and  there- 
fore this  is  a  source  of  warmth  and  not  of  cold.  The  cold  region 
of  Europe  is  to  the  northeast,  over  Russia  and  Siberia. 

Cold  waves  have  different  names  in  different  .countries.  In 
southern  France  the  cold  wind  from  the  north  and  northeast  is 
known  as  the  rais^ra/,  derived  from  the  Latin  word  ma^ister,  mean- 
ing master,  on  account  of  its  strength  and  violence.  In  Russia  the 
name  bur  an  or  purga  is  given  to  the  cold  wave  when  it  blows  along 


76  CONSTRUCTION    OF    WEATHER    MAPS. 

with  it  the  fine  dry  snow  from  the  surface  of  the  ground.  This  buran 
is  apt  to  cause  the  loss  of  many  lives,  both  of  men  and  cattle.  In 
the  Argentine  Republic  the  coolest  wind  is  from  the  southwest. 
It  is  known  as  a  pampero,  from  the  Spanish  pampa,  a  plain. 

Cyclones  and  Anticyclones.  —  A  system  of  winds  blowing  towards 
a  common  center  (such  as  is  well  shown  over  the  Gulf  States  on 
the  weather  map  for  the  second  day,  and  over  the  middle  Atlantic 
coast  on  the  third  day)  is  called  by  meteorologists  a  cyclone.  The 
name  was  first  suggested  by  Piddington  early  in  this  century.  It  is 
derived  from  the  Greek  word  for  circle,  and  hence  it  embodies  the 
idea  of  a  circular  or  spiral  movement  of  the  winds.  A  system  of 
outflowing  winds,  such  as  that  over  the  northwestern  United  States 
shown  on  the  maps  for  the  first  five  days,  and  over  the  western  Gulf 
States  on  the  sixth  day,  is  called  an  anticyclone.  This  name  was 
proposed  by  Galton  in  1863,  and  means  the  opposite  of  cyclone. 


CHAPTER   VII. 

PRESSURE. 

A.  Lines  of  Equal  Pressure :  Isobars.  —  One  of  the  most  impor- 
tant weather  elements  is  the  pressure  of  the  atmosphere.  This 
has  already  been  briefly  discussed  in  the  sections  on  the  mercurial 
barometer  (Chapter  II).  It  was  there  learned  that  atmospheric 
pressure  is  measured  by  the  number  of  inches  of  mercury  which 
the  weight  of  the  air  will  hold  up  in  the  glass  tube  of  the 
barometer.  Our  sensation  of  heat  or  cold  gives  us  some  general 
idea  as  to  the  air  temperature.  We  can  tell  the  wind  direction 
when  we  know  the  points  of  the  compass,  and  can  roughly 
estimate  its  velocity.  No  instrumental  aid  is  necessary  to 
enable  us  to  decide  whether  a  day  is  clear,  fair  or  cloudy,  or 
whether  it  is  raining  or  snowing.  Unlike  the  temperature,  the 
wind,  or  the  weather,  the  pressure  cannot  be  determined  by  our 
own  senses  without  instrumental  aid.  The  next  weather  element 
that  we  shall  study  is  pressure. 

Proceed  as  in  the  case  of  the  thermometer  readings.     Enter 


PRESSURE. 


77 


upon  a  blank  map  the  barometer  readings  for  the  different  stations 
given  in  the  third  column  of  the  table  in  Chapter  VIII.  When 
this  has  been  done  you  have  before  you  the  actual  pressure 
distribution  over  the  United  States  at  7  A.M.,  on  the  first  day 
of  the  series.  Describe  the  distribution  of  pressure  in  general 
terms.  Where  is  the  pressure  highest  ?  Where  lowest  ?  What 
are  the  highest  and  the  lowest  readings  of  the  barometer  noted 
on  the  map  ?  What  is  the  difference  (in  inches  and  hundredth*) 
between  these  readings  ? 

Draw  lines  of  equal  pressure,  following  the  same  principles 
as  were  adopted  in  the  case  of  the  isotherms.  The  latter  were 
drawn  for  every  even  10°  of  temperature.  The  former  are  to 
be  drawn  for  every  even  .10  inch  of  pressure.  Every  station 
which  has  a  barometer  reading  of  an  even  .10  inch  will  be 
passed  through  by  some  line  of  equal  pressure.  Philadelphia, 
Pa.,  with  29.90  must  be  passed  through  by  the  29.90  line  ;  Wil- 
mington, N.  C.,  with  30.00,  must  have  the  30.00  line  passing 
through  it,  etc.  Chicago,  with  30.17  inches,  must  lie  between 


FIG.  32.  — Isobars.    First  Day. 


78  CONSTRUCTION    OF    WEATHER    MAPS. 

the  lines  of  30.10  and  30.20  inches,  and  nearer  the  latter  than 
the  former.  Denver,  Col.,  with  30.35  inches,  must  lie  midway 
between  the  30.30  and  30.40  lines  (Fig.  32). 

Lines  of  equal  pressure  are  called  isobars,  a  word  derived 
from  two  Greek  words  meaning  equal  pressure. 

Describe  the  distribution  of  pressure  as  shown  by  the  arrange- 
ment of  the  isobars.  Note  the  differences  in  form  between  the 
isotherms  and  the  isobars.  The  words  high  and  low  are  printed 
on  weather  maps  to  mark  the  regions  where  the  pressure  is 
highest  and  lowest. 

Draw  isobars  for  the  other  days,  using  the  barometer  read- 
ings given  in  the  table  in  Chapter  VIII.  Figs.  33-38  show  the 
arrangement  of  the  isobars  on  these  days. 

The  pressure  charts  may  be  colored,  as  indicated  by  the  shad- 
ing in  these  figures,  in  order  to  bring  out  more  clearly  the  dis- 
tribution of  pressure,  according  to  the  same  general  scheme  as 
that  adopted  in  the  temperature  charts.  Color  brown  all  parts 
of  your  six  isobaric  charts  over  which  the  pressures  are  below 
29.50  inches  ;  color  red  all  parts  with  pressure  above  30.00 
inches.  Use  a  faint  shade  of  brown  for  pressures  between 
29.50  inches  and  29.00  inches,  and  a  darker  shade  for  pres- 
sures below  29.00  inches.  In  the  case  of  pressures  over 
30.00  inches,  use  a  pale  red  for  pressures  between  30.00 
and  30.50  inches,  and  a  darker  shade  of  red  for  pressures 
above  30.50  inches.  By  means  of  these  colors  the  pressure 
distribution  will  stand  out  very  clearly.  The  scheme  of  color 
and  shading  may,  of  course,  be  varied  to  suit  the  individual 
fancy. 

Study  the  isobaric  chart  of  each  day  of  the  series  by  itself  at 
first.  Describe  the  pressure  distribution  on  each  'chart. 

Then  compare  the  successive  charts.  Note  what  changes 
have  taken  place  in  the  interval  between  each  chart  and  the 
one  preceding ;  where  the  pressures  have  risen ;  where  they 
have  fallen,  and  where  they  have  remained  stationary.  Write 


PRESSURE. 


79 


FIG.  33.  —  Pressure.    First  Day. 


FIG.  34.  — Pressure.    Second  Day. 

a  brief  account  of  the  facts  of  pressure  change  illustrated  on 
the  whole  series  of  six  charts. 


80 


CONSTRUCTION    OF    WEATHER    MAPS. 


FIG.  35.  — Pressure.     Third  Bay. 


FIG.  36.  —  Pressure.    Fourth  Day. 


Compare   the   charts   of   temperature   and   of  pressure,   first 
individually,    then   collectively.     What   relations   do    you  dis- 


PKESSURE. 


81 


FIG.  37. —  Pressure.    Fifth  Day. 


FIG.  38.  — Pressure.    Sixth  Day. 


cover  between  temperature  distribution  and  pressure  distribu- 
tion on  the   isothermal  and  the  isobaric  charts  for  the  same 


82  CONSTRUCTION    OF .  WEATHER    MAPS. 

day  ?  What  relations  can  you  make  out  between  the  changes 
in  temperature  and  pressure  distribution  on  successive  days  ? 
On  the  whole  series  of  maps  ?  Write  out  the  results  of  your 
study  concisely  and  clearly. 

Compare  the  wind  charts  and  the  pressure  charts  for  the  six 
days.  Is  there  any  relation  between  the  direction  and  velocity 
of  the  winds  and  the  pressure  ?  Observe  carefully  the  changes 
in  the  winds  from  day  to  day  on  these  charts,  and  the  changes 
in  pressure  distribution.  Formulate  and  write  out  a  brief  gen- 
eral statement  of  all  the  relations  that  you  have  discovered. 

Mean  Annual  and  Mean  Monthly  Isobaric  Charts.  —  We  have 
thus  far  been  studying  isobaric  charts  based  on  barometer  readings 
made  at  a  single  moment  of  time.  Just  as  there  are  mean  annual 
and  mean  monthly  isothermal  charts,  based  on  the  mean  annual  and 
mean  monthly  temperatures,  so  there  are  mean  annual  and  mean 
monthly  isobaric  charts  for  the  different  countries  and  for  the 
whole  world,  based  on^the  mean  annual  and  mean  monthly  pres- 
sures. The  mean  annual  and  mean  monthly  isobaric  charts  of  the 
world  show  the  presence  of  great  oval  areas  of  low  and  high  pres- 
sure covering  a  whole  continent,  or  a  whole  ocean,  and  keeping 
about  the  same  position  for  months  at  a  time.  Thus,  on  the 
isobaric  chart  showing- the  mean  pressure  over  the  world  in  Jan- 
uary, there  are  seen  immense  area^of  high  pressure  (anticyclones) 
over  the  two  great  continental  masses  of  the  Northern  Hemisphere. 
These  anticyclonic  areas,  although  vastly  greater  in  extent  than  the 
small  ones  seen  on  the  weather  maps  of  the  United  States,  have  the 
same  system  of  spirally  outflowing  winds.  Over  the  northeastern 
portion  of  the  North  Pacific  and  the  North  Atlantic,  in  January, 
are  seen  immense  areas  of  low  pressure  (cyclones)  with  spirally 
inflowing  winds.  In  July  the  northern  continents  are  covered  by 
cyclonic  areas,  and  the  central  portion  of  the  northern  oceans  by 
anticyclonic  areas. 

B>   Direction  and  Rate  of  Pressure  Decrease  :  Pressure  Gradient. 
-  In  Chapter  V  we  studied  the  direction  and  rate  of  tempera- 
ture decrease,  or  temperature  gradient.     We  saw  that  the  direc- 
tion of  this  decrease  varies  in  different  parts  of  the  map,  and  that 


PRESSURE. 


83 


the  rate,  which  depends  upon  the  closeness  of  the  isotherms, 
also  varies.  An  understanding  of  temperature  gradients  makes 
it  easy  to  study  the  directions  and  rates  of  pressure  decrease,  or 
pressure  gradients,  as  they  are  commonly  called.  Examine  the 
series  of  isobaric  charts  to  see  how  the  lines  of  pressure  decrease 
run.  Draw  lines  of  pressure  decrease  for  the  six  isobaric  charts, 
as  you  have  already  done  on  the  isothermal  charts.  When 
the  isobars  are  near  together,  the  lines  of  pressure  decrease 
may  be  drawn  heavier,  to  indicate  a  more  rapid  rate  of  decrease 


FIG.  39.  — Pressure  Gradients.    First  Day. 

of  pressure.  Fig.  39  shows  lines  of  pressure  decrease  for  the 
first  day.  Note  how  the  arrangement  and  direction  of  these 
lines  change  from  one  map  to  the  next.  Compare  these  lines 
with  the  lines  of  temperature  decrease. 

Next  study  the  rate  of  pressure  decrease.  This  rate  depends 
upon  the  closeness  of  the  isobars,  just  as  the  rate  of  temperature 
decrease  depends  upon  the  closeness  of  the  isotherms.  Exam- 
ine the  rates  of  pressure  decrease  upon  the  series  of  isobaric 
charts.  On  which  charts  do  you  find  the  most  rapid  rate? 


84  CONSTRUCTION    OF    WEATHER    MAPS. 

Where  ?  On  which  the  slowest  ?  Where  ?  Do  you  discover 
any  relation  between  rate  of  pressure  decrease  and  the  pressure 
itself  ?  What  relation  ? 

When  expressed  numerically,  the  barometric  gradient  is 
understood  to  mean  the  number  of  hundredths  of  an  inch  of 
change  of  pressure  in  one  latitude  degree.  Prepare  a  scale 
of  latitude  degrees,  and  measure  rates  of  pressure  decrease, 
just  as  you  have  already  done  in  the  case  of  temperature.  In 
this  case,  instead  of  dividing  the  difference  in  temperature 
between  the  isotherms  (10°  =  T)  by  the  distance  between  the 
isobars  (D),  we  substitute  for  10°  of  temperature  .10  inch  of 
pressure  (P).  Otherwise  the  operation  is  precisely  the  same  as 
described  in  Chapter  V.  The  rule  may  be  stated  as  follows  : 
Select  the  station  for  which  you  wish  to  know  the  rate  of 
pressure  decrease  or  the  barometric  gradient.  Lay  your  scale 
through  the  station,  and  as  nearly  as  possible  at  right  angles  to 
the  adjacent  isobars.  If  the  station  is  exactly  on  an  isobar, 
then  measure  the  distance  from  the  station  to  the  nearest  isobar 
indicating  a  lower  pressure.  The  scale  must,  however,  be  laid 
perpendicularly  to  the  isobars,  as  before.  Divide  the  number  of 
hundredths  of  an  inch  of  pressure  difference  between  the  isobars 
(always  .10  inch)  by  the  number  expressing  the  distance  (in  lati- 
tude degrees)  between  the  isobars  ;  the  quotient  is  the  rate  of 
pressure  decrease  per  latitude  degree.  Or,  to  formulate  the 
operation, 


in  which  R  —  rate  ;  P  =  pressure  difference  between  isobare 
(always  .10  inch),  and  D  =  distance  between  the  isobars  in 
latitude  degrees. 

Determine  the  rates  of  pressure  decrease  in  the  following 
cases  :  — 

A.  For  a  number  of  stations  in  different  parts  of  the  same 
map,  as,  e.g.,  Boston,  New  York,  Washington,  Charleston. 


WEATHER.  85 

New  Orleans,  St.  Louis,  St.  Paul,  Denver,  and  on  the  same 
day. 

B.  For  one  station  during  a  winter  month  and  during  a 
summer  month,  measuring  the  rate  on  each  map  throughout 
the  month,  and  obtaining  an  average  rate  for  the  month. 

Have  these  gradients  at  the  different  stations  any  relation  to 
the  proximity  of  low  or  high  pressure  ?  To  the  velocity  of  the 
wind? 

Pressure  Gradients  on  Isobaric  Charts  of  the  Globe,  —  The  change 
from  low  pressure  to  high  pressure  or  vice  versa  with  the  seasons, 
already  noted  as  being  clearly  shown  on  the  isobaric  charts  of  the 
globe,  evidently  means  that  the  directions  of  pressure  decrease  must 
also  change  from  season  to  season.  The  rates  of  pressure  decrease 
likewise  do  not  remain  the  same  all  over  the  world  throughout  the 
year.  If  we  examine  isobaric  charts  for  January  and  July,  we  shall 
find  that  these  gradients  are  stronger  or  steeper  over  the  Northern 
Hemisphere  in  the  former  month  than  in  the  latter. 


CHAPTER    VIII. 

WEATHER. 

HITHERTO  nothing  has  been  said  about  the  weather  itself,  as 
shown  on  the  series  of  maps  we  have  been  studying.  By  weather, 
in  this  connection,  we  mean  the  state  of  the  sky,  whether  it  is 
clear,  fair,  or  cloudy,  or  whether  it  is  raining  or  snowing  at  the 
time  of  the  observation.  While  it  makes  not  the  slightest  dif- 
ference to  our  feelings  whether  the  pressure  is  high  or  low,  the 
character  of  the  weather  is  of  great  importance. 

The  character  of  the  weather  on  each  of  the  days  whose  tem- 
perature, wind,  and  pressure  conditions  we  have  been  studying 
is  noted  in  the  table  in  this  chapter.  The  symbols  used  by  the 
Weather  Bureau  to  indicate  the  different  kinds  of  weather  on 
the  daily  weather  maps  are  as  follows:  Q  clear;  (J  fair,  or 
partly  cloudy;  ^cloudy;  ©rain;  0  snow. 


86  CONSTRUCTION    OF    WEATHER    MAPS. 

Enter  011  a  blank  map,  at  each  station,  the  sign  which  indi- 
cates the  weather  conditions  at  that  station  at  7  A.M.,  011  the 
first  day,  as  given  in  the  table.  When  you  have  completed  this, 
you  have  before  you  on  the  map  a  bird's-eye  view  of  the 
weather  which  prevailed  over  the  United  States  at  the  moment 
of  time  at  which  the  observations  were  taken.  Describe  in 
general  terms  the  distribution  of  weather  here  shown,  naming 
the  districts  or  States  over  which  similar  conditions  prevail. 
Following  out  the  general  scheme  adopted  in  the  case  of  the 
temperature  and  the  pressure  distribution,  separate,  by  means  of 
a  line  drawn  on  your  map,  the  districts  oveF  which  the  weather 
is  prevailingly  cloudy  from  those  over  which  the  weather  is 
partly  cloudy  or  clear.  In  drawing  this  line,  scattering  obser- 
vations which  do  not  harmonize  with  the  prevailing  conditions 
around  them  may  be  disregarded,  as  the  object  is  simply  to 
emphasize  the  general  characteristics.  Enclose  also,  by  means 
of  another  line,  the  general  area  over  which  it  was  snowing 
at  the  time  of  observation,  and  shade  or  color  the  latter 
region  differently  from  the  cloudy  one.  Study  the  weather 
distribution  shown  on  your  chart.  What  general  relation  do 
_you  discover  between  the  kinds  of  weather  and  the  temperature, 
winds,  and  pressure  ? 

Proceed  similarly  with  the  weather  on  the  five  remaining  days, 
as  noted  in  the  table.  Enter  the  weather  symbols  for  each  day 
on  a  separate  blank  map,  enclosing  and  shading  or  coloring  the 
areas  of  cloud  and  of  snow  as  above  suggested.  In  Figs.  40-45 
the  cloudy  areas  are  indicated  by  single-line  shading,  and  the 
snowy  areas  by  double-line  shading. 

Now  study  carefully  each  weather  chart  with  its  corresponding 
temperature,  wind,  and  pressure  charts.  Note  whatever  rela- 
tions you  can  discover  among  the  various  meteorological  ele- 
ments on  each  day.  Then  compare  the  weather  conditions  on 
the  successive  maps.  What  changes  do  you  note  ?  How  are 
these  changes  related  to  the  changes  of  temperature ;  of  wind ; 


WEATHEK. 


87 


FIG.  40. —  Weather.    First  Day. 


FIG.  41.  — Weather.    Second  Day. 


of  pressure  ?     Write   a  summary  of  the  results  derived  from 
your  study  of  these  four  sets  of  charts. 


88 


CONSTRUCTION    OF    WEATHER    MAPS. 


FIG.  42.  — Weather.    Third  Day. 


FIG.  43.  — Weather.    Fourth  Day. 


The  Weather  of  Temperate  and  Torrid  Zones.  —  The  facts  of  the 
presence  of  clear  weather  in  one  region  while  snow  is  falling  in 


WEATHER. 


89 


FIG.  44.  — Weather.    Fifth  Day. 


75° 70° 


FIG.  45.  — Weather.    Sixth  Day. 


another,  and  of  the  variability  of  our  weather  from  day  to  day  in 
different  parts  of  the  United  States,  are  emphasized  by  these  charts 


90  CONSTRUCTION    OF    WEATHEK    MAPS. 

of  weather  conditions.  This  changeableness  of  weather  is  a  marked 
characteristic  of  the  greater  portion  of  the  Temperate  Zones,  espe- 
cially in  winter.  The  weather  maps  for  successive  days  do  not, 
as  a  rule,  show  a  repetition  of  the  same  conditions  over  extended 
regions.  In  the  Torrid  Zone  it  is  different.  Over  the  greater  part 
of  that  zone  the  regularity  of  the  weather  conditions  is  such  that, 
day  after  day,  for  weeks  and  months,  the  same  features  are  repeated. 
There  monotony,  here  variety,  is  the  dominant  characteristic  of  the 
weather. 


PART   IV.  —  THE   CORRELATIONS    OF   THE   WEATHER 
ELEMENTS  AND  WEATHER  FORECASTING. 


CHAPTER   IX. 

CORRELATION   OF   THE   DIRECTION   OF   THE   WIND   AND   THE 
PRESSURE. 

THE  study  of  the  series  of  weather  maps  in  Chapters  V— 
VIII  has  made  it  clear  that  some  fairly  definite  relation  exists 
between  the  general  flow  of  the  winds  and  the  distribution  of 
pressure.  We  now  wish  to  obtain  some  more  definite  result  as 
to  the  relation  of  the  direction  of  the  wind  and  the  pressure. 
In  doing  this  it  is  convenient  to  refer  the  wind  direction  to  the 
barometric  or  pressure  gradient  at  the  station  at  which  the  obser- 
vation is  made.  The  barometric  gradient,  it  will  be  remem- 
bered, is  the  line  along  which  there  is  the  most  rapid  change  of 
pressure,  and  lies  at  right  angles  to  the  isobars  (Chapter  VII). 

Take  a  small  piece  of  tracing  paper,  about  3  inches  square, 
and  draw  upon  it  a  diagram  similar  to  the  one  here  shown. 
Select  the  station  (between  two 


Against- 


left 


M  -with, 


right 


isobars  on  any  weather  map) 
at  which  you  intend  to  make 
your  observation.  Place  the 
center  of  the  tracing  paper 
diagram  over  the  station, 
with  the  dotted  line  along  the 
barometric  gradient,  the  minus  end  of  the  line  being  towards 
the  area  of  low  pressure.  Observe  into  which  of  the  four 
sectors  (marked  right,  left,  with,  against)  the  wind  arrow 

01 


FIG.  46. 


92  WEATHER    ELEMENTS    AND    FORECASTING. 

at  the  station  points.  Keep  a  record  of  the  observation. 
Repeat  the  observation  at  least  100  times,  using  different  sta- 
tions, on  the  same  map  or  on  different  maps.  Tabulate  your 
results  according  to  the  following  scheme,  noting  in  the  first 
column  the  date  of  the  map,  in  the  second,  third,  fourth,  and 
fifth  columns  the  number  of  winds  found  blowing  with,  to  the 
right  or  left  of,  and  against,  the  gradient. 

TABLE  I.  —  CORRELATION    OF    THE    DIRECTION-    OF    THE    WIND 
AND  THE  PRESSURE. 


DATES 

WITH 

RIGHT 

LEFT 

AGAINST 

Sums 

Percentages 

At  the  bottom  of  each  column  write  down  the  number  of 
cases  in  that  column,  and  then  determine  the  percentages  which 
these  cases  are  of  the  total  number  of  observations.  This  is 
done  by  dividing  the  number  of  cases  in  each  column  by  the 
sum-total  of  all  the  observations.  When  you  have  obtained 
the  percentage  of  each  kind  of  wind  direction,  you  have  a 
numerical  result. 

A  graphical  presentation  of  the  results  may  be  made  by  lay- 
ing off  radii  corresponding  in  position  to  those  which  divide  the 
sectors  in  Fig.  46,  and  whose  lengths  are  proportionate  to  the 
percentages  of  the  different  Avind  directions  in  the  table.  Thus, 
for  a  percentage  of  20,  the  radii  may  be  made  1  inch  long,  for 
40%,  2  inches,  etc.  When  completed,  the  relative  sizes  of 
the  sectors  will  show  the  relative  frequencies  of  winds  blowing 
in  the  four  different  directions  with  reference  to  the  gradient, 
as  is  indicated  in  Fig.  47. 


VELOCITY    OF    WIND    AND    PRESSURE.  93 

The  Deflection  of  the  Wind  from  the  Gradient:  Ferrel's  Law.— 
The  law  of  the  deflection  of  the  wind  prevailingly  to  the  right  of 
the  gradient  is  known  as  FerreVs  Law,  after  William  Ferrel,  a  noted 
American  meteorologist,  who  died  in  1891.  The  operation  of  this 
law  has  already  been  seen  in  the  spiral  circulation  of  the  winds 
around  the  cyclone  and  the  anti- 
cyclone, as  shown  on  the  maps  of 
our  series.  In  the  case  of  the  cy- 
clone the  gradient  is  directed  inward 
towards  the  center ;  in  the  case  of  the 
anticyclone  the  gradient  is  directed 
outward  from  the  center.  In  both 

cases  the  right-handed  deflection  results  in  a  spiral  whirl,  inward  in 
the  cyclone,  outward  in  the  anticyclone.  The  operation  of  this  law  is 
further  seen  in  the  case  of  the  Northeast  Trade  Winds.  These 
winds  blow  from  about  Lat.  30°  N.  towards  the  equator,  with  won- 
derful regularity,  especially  over  the  oceans.  Instead  of  following 
the  gradient  and  blowing  as  north  winds,  these  trades  turn  to  the 
right  of  the  gradient  and  become  northeast  winds,  whence  their 
name.  From  about  Lat.  30°  N.  towards  the  North  Pole  there  is 
another  great  flow  of  winds  over  the  earth's  surface.  These  winds 
do  not  flow  due  north,  as  south  winds.  They  turn  to  the  right,  as 
do  the  trades,  and  become  southwest  or  west-southwest  winds,  being- 
known  as  the  Prevailing  Westerlies.  Ferrel's  Law  thus  operates  in 
the  larger  case  of  the  general  circulation  of  the  earth's  atmosphere, 
as  well  as  in  the  smaller  case  of  the  local  winds  on  our  weather  maps. 


CHAPTER   X. 

CORRELATION  OF  THE  VELOCITY  OF  THE  WIND  AND 
THE  PRESSURE. 

PREPARE  a  scale  of  latitude  degrees,  as  explained  in  Chap- 
ter V.  Select  some  station  on  the  weather  map  at  which  there 
is  a  wind  arrow,  and  at  which  you  wish  to  study  the  relation  of 
wind  velocity  and  pressure.  Find  the  rate  of  pressure  change 
per  degree  as  explained  in  Chapter  VII.  Note  also  the  velocity, 
in  miles  per  hour,  of  the  wind  at  the  station.  Repeat  the 


94 


WEATHER    ELEMENTS    AND   FORECASTING. 


operation  100  or  more  times,  selecting  stations  in  different 
parts  of  the  United  States.  It  is  well,  however,  to  include  in 
one  investigation  either  interior  stations  alone  (i.e.,  more  than 
100  miles  from  the  coast)  or  coast  stations  alone,  as  the 
wind  velocities  are  often  considerably  affected  by  proximity  to 
the  ocean.  And,  if  coast  stations  are  selected,  either  onshore 
or  offshore  winds  should  alone  be  included  in  one  exercise. 
The  investigation  may,  therefore,  be  carried  out  so  as  to 
embrace  the  following  different  sets  of  operations :  — 

A.  Interior  stations. 

B.  Coast  stations  with  onshore  winds. 
O.    Coast  stations  with  offshore  winds. 

Enter  your  results  in  a  table  similar  to  the  one  here  given :  — 

TABLE  II.  —  CORRELATION  OF  WIND  VELOCITY  AND  BAROMETRIC 

GRADIENT. 

For  interior  (or  coast)  stations,  with  onshore  (or  offshore)  winds,  in  the 
United  States  during  the  month  (or  months)  of 


Hates  of  Pressure 
Change  per  Lati- 
tude Degree 

oo-20 

20-10 

10-5 

5-3| 

3*-2i 

' 
2£-2 

. 
etc. 

.Distances  between 
Isobars  in  Lati- 
tude Degrees 

<H 

*-l 

1-2 

2-3 

3-4 

4-5 

etc. 

Wind  Velocities 
(miles  per  hour) 

Sums 

Cases 

Means 

VELOCITY    OF    WIND   AND   PRESSURE.     ,  95 

The  wind  velocity  for  each  station  is  to  be  entered  in  the 
column  at  whose  top  is  the  rate  of  pressure  change  found  for 
that  station.  Thus,  if  for  any  station  the  rate  of  pressure  change 
is  3£  (i.e.,  .03  inch  in  one  latitude  degree),  and  the  wind  velocity 
at  that  station  is  IT  miles  an  hour,  enter  the  17  in  the  fourth 
and  fifth  columns  of  the  table.  When  you  find  that  the  rate  of 
pressure  change  for  any  station  falls  into  two  columns  of  the 
table,  as,  e.g.,  10,  or  5,  or  31,  then  enter  the  corresponding  wind 
velocity  in  both  those  columns. 

In  the  space  marked  Sums  write  the  sum-total  of  all  the  wind 
velocities  in  each  column.  The  Cases  are  the  number  of  sepa- 
rate observations  you  have  in  each  column.  The  Means  denote 
the  average  or  mean  wind  velocities  found  in  each  column,  and 
are  obtained  by  dividing  the  sums  by  the  cases. 

Study  the  results  of  your  table  carefully.  Deduce  from  your 
own  results  a  general  rule  for  wind  velocities  as  related  to 
barometric  gradients. 

The  dependence  of  wind  velocities  on  the  pressure  gradient  is  a 

fact  of  gr.eat  importance  in  meteorology.  The  ship  captain  at  sea 
knows  that  a  rapid  fall  of  his  barometer  means  a  rapid  rate  of  pres- 
sure change,  and  foretells  high  winds.  He  therefore  makes  his 
preparations  accordingly,  by  shortening  sail  and  by  making  every- 
thing fast.  The  isobaric  charts  of  the  globe  for  January  and  July 
show  that  the  pressure  gradients  are  stronger  (i.e.,  the  rate  of 
pressure  change  is  more  rapid)  over  the  Northern  Hemisphere  in 
January  than  in  July.  This  fact  would  lead  us  to  expect  that  the 
velocities  of  the  general  winds  over  the  Northern  Hemisphere  should 
be  higher  in  winter  than  in  summer,  and  so  they  are.  Observations 
of  the  movements  of  clouds  made  at  Blue  Hill  Observatory,  Hyde 
Park,  Mass.,  show  that  the  whole  atmosphere,  up  to  the  highest 
cloud  level,  moves  almost  twice  as  fast  in  winter  as  in  summer.  In 
the  higher  latitudes  of  the  Southern  Hemisphere,  where  the  baro- 
metric gradients  are  prevailingly  much  stronger  than  in  the  North- 
ern, the  wind  velocities  are  also  prevailingly  higher  than  they  are 
north  of  the  equator.  The  prevailing  westerly  winds  of  the  South- 
ern Hemisphere,  south  of  latitude  of  30°  S.,  blow  with  high  veloci- 


96  WEATHER    ELEMENTS    AND    FORECASTING. 

ties  nearly  all  the  time,  especially  during  the  winter  months  (June. 
July,  August).  These  winds  are  so  strong  from  the  westward  that 
vessels  trying  to  round  Cape  Horn  from  the  east  often  .occupy  weeks 
beating  against  head  gales,  which  continually  blow  them  back  011 
their  course. 


CHAPTER    XI. 

FORM   AND   DIMENSIONS    OF   CYCLONES    AND   ANTICYCLONES. 

A.  Cyclones. — Provide  yourself  with  a  sheet  of  tracing  paper 
about  half  as  large  as  the  daily  weather  map.  Draw  a  straight 
line  across  the  middle  of  it ;  mark  a  dot  at  the  center  of  the 
line,  the  letter  N  at  one  end,  and  the  letter  >S  at  the  other. 
Place  the  tracing  paper  over  a  weather  map  on  which  there  is 
a  fairly  well  enclosed  center  of  low  pressure  (low),  having  the 
dot  at  the  center  of  the  low,  and  the  line  parallel  to  the  near- 
est meridian,  the  end  marked  N  being  towards  the  top  of  the 
map.  When  thus  placed,  the  paper  is  said  to  be  oriented.  Trace 
off  the  isobars  which  are  nearest  the  center.  In  most  cases  the 
29.80-inch  isobar  furnishes  a  good  limit,  out  to  which  the 
isobars  may  be  traced.  Continue  this  process,  using  different 
weather  maps,  until  the  lines  on  the  tracing  paper  begin  to 
become  too  confused  for  fairly  easy  seeing.  Probably  15  or  20 
separate  areas  of  low  pressure  may  be  traced  on  to  the  paper. 
It  is  important  to  have  all  parts  of  the  cyclonic  areas  repre- 
sented on  your  tracing.  If  most  of  the  isobars  you  have  traced 
are  on  the  southern  side  of  cyclones  central  over  the  Lakes  or 
lower  St.  Lawrence,  so  that  the  isobars  on  the  northern  sides 
are  incomplete,  select  for  your  further  tracings  weather  maps 
on  which  the  cyclonic  centers  are  in  the  central  or  southern 
portions  of  the  United  States,  and  therefore  have  their  northern 
isobars  fully  drawn. 

When  your  tracing  is  finished  you  have  a  composite  portrait 
of  the  isobars  around  several  areas  of  low  pressure.  Now  study 


CYCLONES    AND    ANTICYCLONES.  97 

the  results  carefully.  Draw  a  heavy  pencil  or  an  ink  line  on 
the  tracing  paper,  in  such  a  way  as  to  enclose  the  average  area 
outlined  by  the  isobars.  This  average  area  will  naturally  be  of 
smaller  dimensions  than  the  outer  isobars  011  the  tracing  paper, 
and  of  larger  dimensions  than  the  inner  isobars,  and  its  form 
will  follow  the  general  trend  indicated  by  the  majority  of  the 
isobars,  without  reproducing  any  exceptional  shapes. 

Write  out  a  careful  description  of  the  average  form,  dimen- 
sions [measured  by  a  scale  of  miles  or  of  latitude  degrees  (70 
miles  =  1  degree  about)]  and  gradients  of  these  areas  of  low 
pressure,  noting  any  tendency  to  elongate  in  a  particular  direc- 
tion; any  portions  of  the  composite  where  the  gradients  are 
especially  strong,  weak,  etc. 

B.  Anticyclones.  —  This  investigation  is  carried  out  in  pre-- 
cisely  the  same  manner  as  the  preceding  one,  except  that  anti- 
cyclones (highs)  are  now  studied  instead  of  cyclones.  The 
isobars  may  be  traced  off  as  far  away  from  the  center  as  the 
30.20-inch  line  in  most  cases.  When,  however,  the  pressure  at 
the  center  is  exceptionally  high,  it  will  not  be  necessary  to  trace 
off  lower  isobars  than  those  for  30.30,  or  30.40,  or  sometimes 
30.50  inches. 

Loomis's  Results  as  to  Form  and  Dimensions  of  Cyclones  and 
Anticyclones.  —  One  of  the  leading  American  meteorologists,  Loomis, 
who  was  for  many  years  a  professor  in  Yale  University,  made  an 
extended  study  of  the  form  and  dimensions  of  areas  of  low  and  high 
pressure  as  they  appear  on  our  daily  weather  maps.  In  the  cases 
of  areas  of  low  pressure  which  he  examined,  the  average  form  of 
the  areas  was  elliptical,  the  longer  diameter  being  nearly  twice  as 
long  as  the  shorter  (to  be  exact  the  ratio  was  1.94: 1).  The  average 
direction  of  the  longer  diameter  he  found  to  be  about  NE.  (K  36° 
E.),  and  the  length  of  the  longer  diameter  often  1600  miles.  In  the 
case  of  areas  of  high  pressure,  Loomis  also  found  an  elliptical  form 
predominating;  the  longer  diameter  being  about  twice  as  long  as 
the  shorter  (ratio  1.91:1),  and  the  direction  of  trend  about  NE. 
(N.  44°  E.).  These  characteristics  hold,  in  general,  for  the  cyclonic 


98  WEATHER    ELEMENTS    AND    FORECASTING. 

and  anticyclonic  areas  of  Europe  also.  The  cyclones  of  the  tropics 
differ  considerably  from  those  of  temperate  latitudes  in  being  nearly 
circular  in  form. 


CHAPTER    XII. 

CORRELATION    OF    CYCLONES    AND    ANTICYCLONES    WITH    THEIR 
WIND   CIRCULATION. 

A.  Cyclones.  —  Something  as  to  the  control  of  pressure  over 
the  circulation  of  the  wind  has  been  seen  in  the  preliminary  exer- 
cises on  the  daily  weather  maps.  We  now  proceed  to  investi- 
gate this  correlation  further  by  means  of  the  composite  portrait 
method.  This  method  is  a  device  to  bring  out  more  clearly 
the  general  systems  of  the  winds  by  throwing  together  on  to  one 
sheet  a  large  number  of  wind  arrows  in  their  proper  position 
with  reference  to  the  controlling  center  of  low  pressure.  In 
this  way  we  have  many  more  observations  to  help  us  in  our 
investigation  than  if  we  used  only  those  which  are  given  on  one 
weather  map,  and  the  circulation  can  be  much  more  clearly 
made  out. 

Provide  yourself  with  a  sheet  of  tracing  paper,  prepared  as 
described  in  Chapter  XL  Place  the  paper  over  an  area  of 
low  pressure  on  some  weather  map,  with  the  dot  at  the  center 
of  the  low,  and  having  the  paper  properly  oriented,  as  already 
explained.  Trace  off  all  the  wind  arrows  around  the  center  of 
low  pressure,  making  the  lengths  of  these  arrows  roughly  pro- 
portionate (by  eye)  to  the  velocity  of  the  wind,  according  to 
some  scale  previously  determined  upon.  Include  on  your  trac- 
ing all  the  wind  arrows  reported  at  stations  whose  lines  of 
pressure-decrease  converge  towards  the  low  pressure  center. 
Repeat  this  operation,  using  other  centers  of  low  pressure  on 
other  maps,  until  the  number  of  arrows  on  the  tracing  paper  is 
so  great  that  the  composite  begins  to  become  confused.  Be 
careful  always  to  center  and  orient  your  tracing  paper  properly. 


AVIND    CIRCULATION.  99 

Select  the  weather  maps  from  which  you  take  your  wind  arrows 
so  that  the  composite  shall  properly  represent  winds  in  all  parts 
of  the  cyclonic  area. 

Deduce  a  general  rule  for  the  circulation  and  velocity  of  the 
wind  in  a  cyclonic  area,  as  shown  on  your  tracing,  and  write  it 
out. 

B.  Anticyclones.  —  This  exercise  is  done  in  precisely  the  same 
way  as  the  preceding  one,  except  that  anticyclones  and  their 
winds  are  studied  instead  of  cyclones. 

Deduce  a  general  rule  for  the  circulation  and  velocity  of  the 
wind  in  an  anticyclonic  area,  as  shown  on  your  tracing,  and 
write  it  out. 

The  control  of  the  wind  circulation  by  areas  of  low  and  high 
pressure  is  one  of  the  most  important  laws  in  meteorology.  Buys- 
Ballot,  a  Dutch  meteorologist,  first  called  attention  to  the  impor- 
tance of  this  law  in  Europe,  and  it  has  ever  since  been  known  by 
his  name.  Buys-Ballot's  Law  is  generally  stated  as  follows  :  Stand 
with  your  back  to  the  wind,  and  the  barometer  will  be  lower  on  your  left 
hand  than  on  your  right.1  This  statement,  as  wih1  be  seen,  covers 
both  cyclonic  and  anticyclonic  systems.  The  circulations  shown  on 
your  tracings  hold  everywhere  in  tlie  Northern  Hemisphere,  not 
only  around  the  areas  of  low  and  high  pressure  seen  on  the  United 
States  weather  maps,  but  around  those  which  are  found  in  Europe 
and  Asia,  and  over  the  oceans  as  well.  Mention  has  already  been 
made,  in  the  chapter  on  isobars  (VII),  of  the  occurrence  of  immense 
cyclonic  and  anticyclonic  areas,  covering  the  greater  portion  of  a 
continent  or  an  ocean,  and  lasting  for  months  at  a  time.  These 
great  cyclones  and  anticyclones  have  the  same  systems  of  winds 
around  them  that  the  smaller  areas,  with  similar  characteristics, 
have  on  our  weather  maps.  A  further  extension  of  what  has  just 
been  learned  will  show  that  if  in  any  region  there  comes  a  change 
from  low  pressure  to  high  pressure,  or  vice  versa,  the  system  of 
winds  in  that  region  will  also  change.  Many  such  changes  of  pres- 
sures and  winds  actually  occur  in  different  parts  of  the  world,  and 
are  of  great  importance  in  controlling  the  climate.  The  best-known 
and  the  most-marked  of  all  these  changes  occurs  in  the  case  of 
1  In  the  Northern  Hemisphere. 


100  WEATHER    ELEMENTS    AND    FORECASTING. 

India.  During  the  winter,  an  anticyclonic  area  of  high  pressure 
is  central  over  the  continent  of  Asia.  The  winds  blow  out  from  it 
on  all  sides,  thus  causing  general  northeasterly  winds  over  the 
greater  portion  of  India.  These  winds  are  prevailingly  dry  and 
clear,  and  the  weather  during  the  time  they  blow  is  fine.  India 
then  has  its  dry  season.  As  the  summer  comes  on,  the  pressure 
over  Asia  changes,  becoming  low;  a  cyclonic  area  replaces  the 
winter  anticyclone,  and  inflowing  winds  take  the  place  of  the  out- 
flowing ones  of  the  winter.  The  summer  winds  cross  India  from  a 
general  southwesterly  direction,  come  from  over  the  ocean,  and  are 
moist  and  rainy.  India  then  has  its  rainy  season.  These  seasonal 
winds  are  known  as  Monsoons,  a  name  derived  from  the  Arabic  and 
meaning  seasonal. 

The  accompanying  figure  (Fig.  48)  is  taken  from  the  Pilot  Chart 
of  the  North  Atlantic  Ocean,  published  by  the  Hydrographic  Office 
*  of  the  United  States  Navy  for  the  use 

.  /  jf  +f/       ..  of  seamen.     It  shows  the  wind  circular 

.  I   X     'j     ij    .•••'"  tion   around   the  center   of   a   cyclone 

1     \,)(  ^vVc""""^^     which  is  moving  northward  along  the 

-,\ '  '''-I V-./k'ffk'''".-^ X..    Atlantic  Coast   of   the  United   States. 

The  long  arrow  indicates  the  path  of 
movement ;  the  shorter  arrows  indicate 
the  directions  of  the  winds.  By  means 
of  such  a  diagram  as  this  a  captain  is 
able  to  calculate,  with  a  considerable 
degree  of  accuracy,  the  position  of 
the  center  of  the  cyclone,  and  can 
often  avoid  the  violent  winds  near 

that  center  by  sailing  away  from  it,  or  by  "lying  to,"  as  it 
is  called,  and  waiting  until  the  center  passes  by  him  at  a  safe 
distance.  These  cyclones  which  come  up  the  eastern  coast  of  the 
United  States  at  certain  seasons  are  usually  violent,  and  often 
do  considerable  damage  to  shipping.  The  Weather  Bureau  gives 
all  the  warning  possible  of  the  coming  of  these  hurricanes,  as  they 
are  called,  by  displaying  hurricane  signals  along  the  coast,  and  by 
issuing  telegraphic  warnings  to  newspapers.  In  this  way  ship  cap- 
tains, knowing  of  the  approach  of  gales  dangerous  to  navigation, 
may  keep  their  vessels  in  port  until  all  danger  is  past.  Millions  of 
dollars'  worth  of  property  and  hundreds  of  lives  have  thus  been  saved. 


.•J      •      .         y  f^  .» _^_  .••*^V 

f.         y*4     ://  ^-       ^ 

«s 


./ 


DIRECTION    OF    WIND    AND    TEMPERATURE. 


101 


CHAPTER    XIII. 


CORRELATION  OF  THE  DIRECTION  OF  THE  WIND  AND  THE 
TEMPERATURE. 

IT  is  evident,  from  even  the  most  general  observation  of  the 
weather  elements,  that  the  temperature  experienced  at  any 
place  is  very  largely  dependent  upon  the  direction  of  the  wind. 
Thus,  for  instance,  in  the  United  States,  a  wind  from  some 
northerly  point  is  likely  to  bring  a  lower  temperature  than  a 
southerly  wind.  To  investigate  this  matter  more  closely,  and 
to  discover  how  the  winds  at  any  station  during  any  month  are 
related  to  the  temperatures  noted  at  that  station,  we  proceed 
as  follows  :  — 

Select  the  Weather  Bureau  station  at  which  you  wish  to 
study  these  conditions.  Note  the  direction  of  the  wind  and 
the  temperature  at  that  station  on  the  first  day  of  any  month. 
Prepare  a  table  similar  to  the  following  one. 

TABLE  III.  —  CORRELATION  OF  THE  DIRECTION  OF  THE  WIND 
AND  THE  TEMPERATURE. 

At  during  the  Month  of  


WIND 
DIRECTIONS 

N. 

NE. 

E. 

SE. 

S. 

sw. 

w. 

NW. 

TEMPERA- 

TUBES 

Sums 

Total 

Cases 

Total 

Means 

Mean 

102 


WEATHER    ELEMENTS    AND    FORECASTING. 


c  I£nter  the  temperature  at  8  A.M.  on  the  first  day  of  the  month 
%  ^iL  vthe   table   under  the    proper  wind   direction. 


•a- 

Thus,  if  the  wind  is  NE.,  and  the  temperature  42°,  enter  42  in 
the  second  column  of  the  table.  Repeat  the  observation  for  the 
same  station,  and  for  all  the  other  days  of  the  month,  recording 
the  temperatures  in  each  case  in  their  appropriate  columns  in 
the  table.  Omit  all  cases  in  which  the  wind  is  light,  because 
winds  of  low  velocities  are  apt  to  be  considerably  affected  by  local 
influences.  When  the  observations  for  the  whole  month  have 
been  entered  in  the  table,  add  up  all  the  temperatures  in  each 
column  (sums).  Find  the  mean  temperature  (means)  observed 
with  each  wind  direction  by  dividing  the  sums  by  the  number  of 
observations  in  each  column  (cases).  Add  all  the  sums  together  ; 
divide  by  the  total  number  of  cases,  and  the  result  will  be  the 
mean  temperature*  for  the  month  at  the  station.  The  general 

effect   of  the  different  wind 
•M  directions  upon  the  tempera- 

ture is  shown  by  a  comparison 
of  the  means  derived  from 
each  column  with  the  mean 
for  the  month. 

A  graphic  representation 
of  the  results  of  this  investiga- 
tion will  help  to  emphasize 
the  lesson.  Draw,  as  in  the 
accompanying  figure  (Fig.  49), 
eight  lines  from  a  central 
point,  each  line  to  represent 
one  of  the  eight  wind  direc- 
tions. About  the  central  point  describe  a  circle,  the  length  of 
whose  radius  shall  correspond  to  the  mean  temperature  of  the 
month,  measured  on  some  convenient  scale.  Thus,  if  the  mean 


SW- 


*  Derived  from  the  8  A.M.  observations.     This  does  not  give  the  true  mean 
temperature. 


DIRECTION    OF    WIND    AND    TEMPERATURE.  103 

temperature  of  the  month  is  55°  and  a  scale  of  half  an  inch  is 
taken  to  correspond  to  10°  of  temperature,  the  radius  of  the 
circle  must  be  five  and  a  half  times  half  an  inch,  or  2|  inches. 
Next  lay  off  on  the  eight  wind  lines  the  mean  temperatures 
corresponding  to  the  eight  different  wind  directions,  using  the 
same  scale  (i  in.  =  10°)  as  in  the  previous  case.  Join  the 
points  thus  laid  off  by  a  heavy  line,  as  shown  in  Fig.  49.  The 
figure,  when  completed,  gives  at  a  glance  a  general  idea  of 
the  control  exercised  by  the  winds  over  the  temperatures  at 
the  station  selected.  Where  the  heavy  line  crosses  a  wind  line 
inside  the  circle  it  shows  that  the  average  temperature  accom- 
panying the  corresponding  wind  direction  is  below  the  mean. 
When  the  heavy  line  crosses  any  wind  line  outside  the  circle, 
it  shows  that  the  average  temperature  accompanying  the  cor- 
responding wind  direction  is  above  the  mean.  Such  a  figure 
is  known  as  a  wind  rose. 

The  cold  wave  and  the  sirocco  are  two  winds  which  exercise 
marked  controls  over  the  temperature  at  stations  in  the  central  and 
eastern  United  States.  The  cold  wave  has  already  been  described 
in  Chapter  V.  It  is  a  characteristic  feature  of  our  winter  weather. 
It  blows  down  from  our  Northwestern  States  or  from  the  Canadian 
Northwest,  on  the  western  side  of  a  cyclone.  It  usually  causes 
sudden  and  marked  falls  in  temperature,  sometimes  amounting  to 
as  much  as  50°  in  24  hours.  The  sirocco  is  a  southerly  or  south- 
westerly wind.  It  also  blows  into  a  cyclone,  but  on  its  southern  or 
southeastern  side.  Coming  from  warmer  latitudes,  and  from  over 
warm  ocean  waters,  the  sirocco  is  usually  a  warm  wind,  in  marked 
contrast  to  the  cold  wave.  In  winter,  in  the  Mississippi  Valley  and 
on  the  Atlantic  Coast,  the  sirocco  is  usually  accompanied  by  warm, 
damp,  cloudy,  and  snowy  or  rainy  weather.  The  high  temperatures 
accompanying  it  (they  may  be  as  high  as  50°  or  60°  even  in  mid- 
winter) are  very  disagreeable.  Our  warm  houses  and  our  winter 
clothing  become  oppressive  and  we  long  for  the  bright,  crisp,  cold 
weather  brought  by  the  cold  wave.  In  summer  when  a  sirocco  blows 
we  have  our  hottest  spells.  Then  sunstrokes  and  prostrations 
by  the  heat  are  most  common,  and  our  highest  temperatures  are 


104  WEATHER    ELEMENTS    AND    FORECASTING. 

recorded.  The  word  sirocco  (from  Syriacus  —  Syrian)  was  first  used 
as  the  name  of  a  warm  southerly  wind  in  Italy.  The  cause  and  the 
characteristics  of  the  Italian  sirocco  and  of  the  American  sirocco 
are  similar,  and  the  name  may  therefore  be  applied  to  our  wind  as 
well  as  to  the  Italian  one.  In  the  Southern  Hemisphere,  at  Buenos 
Ayres,  Argentine  Bepublic,  there  is  a  similar  contrast  between  two 
different  winds.  The  pampero  is  similar  in  many  respects  to  our 
cold  wave.  It  is  a  dry,  cool,  and  refreshing  wind,  blowing  over  the 
vast  level  stretches  of  the  Argentine  pampas  from  the  southwest. 
The  norte  is  a  warm,  damp,  depressing  northerly  wind  corresponding 
to  our  sirocco. 


CHAPTER    XIV. 

CORRELATION   OF   CYCLONES   AND  ANTICYCLONES   AND   THEIR 
TEMPERATURES. 

A.  Cyclones.  —  It  follows  from  the  two  preceding  exercises 
that  some  fairly  definite  distribution  of  temperature,  depending 
upon  the  wind  direction,  should  exist  around  areas  of  low  and 

high  pressure.  Try  to  predict,  on 
the  basis  of  the  results  obtained  in 

NW/  I  ^f£.         Chapters  XII  and  XIII,  what  this 

relation  of  temperatures  and  cy- 
clones and  anticyclones  is.  Then 
work  out  the  relation  independ- 
ently of  your  prediction,  by  study- 
ing actual  cases  obtained  from  the 
weather  maps,  as  follows  :  — 

Prepare  a  sheet  of  tracing 
paper  as  shown  in  Fig.  50.  The 
diameter  of  the  circle  should  be  sufficiently  large  to  include 
within  the  circle  the  average  area  covered  by  a  cyclone  on 
the  weather  maps.  Place  the  tracing  paper,  properly  divided 
in  accordance  with  the  figure,  over  a  well-defined  area  of  low 
pressure  on  a  weather  map,  centering  and  orienting  it  carefully. 


CYCLONES    AND    ANTICYCLONES.  105 

Take  the  temperature  at  the  station  which  lies  nearest  the  center 
of  the  figure  as  the  standard.  Notice  the  temperatures  at  all  the 
other  stations  which  fall  within  the  limits  of  the  circle,  and 
mark  down  at  the  proper  places  on  the  tracing  paper,  the  -f  or  — 
departures  of  these  local  temperatures  from  the  standard  tem- 
perature. Thus,  if  the  standard  is  37°,  and  a  station  has  a 
temperature  of  46°,  enter  +  9°  at  the  proper  place  on  your 
tracing  paper.  Again,  if  a  certain  station  has  24°,  enter  —  13° 
at  the  proper  place  on  the  paper.  Continue  this  process  until 
your  paper  has  all  of  its  divisions  well  filled.  It  is  best  to 
select  all  the  maps  used  in  this  investigation  from  the  same 
month,  for  in  that  case  the  data  are  more  comparable  than  if 
different  months  are  taken.  When  a  sufficient  number  of  ex- 
amples has  been  obtained,  find  the  average  departure  (+  or  — ) 
of  the  temperatures  in  each  division  of  the  tracing  from  the 
central  standard  temperature.  Express  these  averages  graph- 
ically by  means  of  a  wind  rose,  as  in  the  last  exercise. 

Another  Method.  —  The  above  correlation  may  be  investigated 
by  means  of  another  method,  as  follows  :  - 

Prepare  a  piece  of  tracing  paper  by  drawing  an  N.  and  S.  line 
upon  it,  and  placing  a  dot  at  the  center  of  the  line.  Lay  the 
paper  over  an  area  of  low  pressure  on  any  weather  map,  center- 
ing and  orienting  it  properly,  as  in  the  previous  exercises. 
Trace  off  the  isotherms  which  are  near  the  center  of  low  pressure. 
Repeat  this  process  with  several  maps,  selecting  different  ones 
from  those  used  in  the  first  part  of  this  exercise.  Formulate 
a  rule  for  the  observed  distribution  of  temperature,  and  deter- 
mine the  reasons  for  this  distribution.  Note  carefully  any 
effects  of  the  cyclone  upon  the  temperature  gradient. 

B.  Anticyclones.  —  The  correlation  of  anticyclones  with  their 
temperatures  is  studied  in  precisely  the  same  way  as  the  preced- 
ing correlation.  Both  methods  suggested  in  the  case  of  cyclones 
should  be  used  in  the  case  of  anticyclones.  When  your  results 
have  been  obtained,  formulate  a  general  rule  for  the  observed 


106  WEATHER    ELEMENTS    AND    FORECASTING. 

distribution  of  temperature  in  anticyclones,  and  determine  the 
reasons  for  this  distribution. 

Find  from  your  composites  the  average  temperature  of  cy- 
clones and  of  anticyclones,  and  compare  these  averages. 

The  unsymmetrical  distribution  of  temperature  around  cyclones, 

which  is  made  clear  by  the  foregoing  exercises,  is  very  characteristic 
of  these  storms  in  our  latitudes,  and  especially  in  the  eastern  United 
States.  That  this  has  an  important  effect  upon  weather  changes  is 
evident,  and  will  be  further  noted  in  the  chapter  on  Weather  Fore- 
casting. The  cyclones  which  begin  over  the  oceans  near  the  equa- 
tor at  certain  seasons,  and  thence  travel  to  higher  latitudes,— 
tropical  cyclones,  so  called,  —  differ  markedly  from  our  cyclones  in 
respect  to  the  distribution  of  temperature  around  them.  The  tem- 
peratures on  all  sides  of  tropical  cyclones  are  usually  remarkably 
uniform,  the  isotherms  coinciding  fairly  closely  with  the  isobars. 
The  reason  for  this  is  to  be  found  in  the  remarkable  uniformity 
of  the  temperature  and  humidity  conditions  over  the  surrounding 
ocean  surface,  from  which  the  inflowing  winds  come.  In  the 
case  of  our  own  cyclones,  in  the  eastern  United  States,  the  warm 
southerly  wind,  or  sirocco,  in  front  of  the  center  has  very  different 
characteristics  from  those  of  the  cold  northwesterly  wind,  or  cold 
ivave,  in  the  rear,  as  has  become  evident  through  the  preceding 
exercise.  These  winds,  therefore,  naturally  show  their  effects  in 
the  distribution  of  the  temperatures  in  different  parts  of  the  cyclonic 
area. 


CHAPTER    XV. 

CORRELATION   OF   THE   DIRECTION   OF   THE   WIND   AND   THE 

WEATHER. 

SELECT  a  file  of  daily  weather  maps  for  some  month.  Com- 
mencing with  the  first  map  in  the  set,  observe  the  weather  and 
the  direction  of  the  wind  at  a  considerable  number  of  stations 
in  the  same  general  region  (as,  e.g.,  the  Lake  region,  the  lower 
Mississippi  Valley,  the  Pacific  Coast,  etc.).  Enter  each  case  in 
a  table,  similar  to  Table  IV  below,  by  making  a  check  in  the 


DIRECTION    OF    WIND    AND    WEATHER. 


107 


column  under  the  appropriate  wind  direction  and  on  a  line  with 
the  appropriate  type  of  weather. 

TABLE  IV.  —  CORRELATION  OF  THE  DIRECTION  OF  THE  WIND 
AND  THE  WEATHER. 


At  ... 


during  the  Month  of 


N. 

NE. 

E. 

SE. 

S. 

sw. 

w. 

NW. 

TOTALS 

PER- 
CENTAGES 

Clear 

D 

J 

Fair 

F 

K 

Cloudy 

G 

L 

Rain  and  Snow 

H 

M 

Totals 

A 

Percentages 

B 

C 

etc. 

Count  every  observation  of  rain  or  snow  also  as  cloudy,  for  it 
usually  rains  or  snows  only  when  the  sky  is  cloudy.  Continue 
your  observations  on  all  the  maps  for  the  month  you  have 
chosen.  Then  count  up  the  whole  number  of  cases  of  clear 
weather  you  have  found  with  north  winds,  and  write  down  this 
sum  in  the  first  space,  in  the  column  reserved  for  N.  winds. 
Do  the  same  with  fair  and  cloudy  weather.  Add  up  and  enter 
at  the  bottom  of  the  column  in  the  space  marked  Totals  the 
whole  number  of  observations  of  clear,  fair,  and  cloudy  weather 
you  have  observed  with  N.  winds.  Then  find  what  percentage  of 
the  weather  with  N.  winds  was  clear,  and  enter  this  percentage 
next  to  the  sum  of  clear  weather  observations,  in  the  first  divi- 
sion in  the  column  headed  N.  Do  the  same  for  fair,  cloudy, 
and  rainy  or  snowy  weather,  deriving  the  percentages  of  rain 
or  snow  from  the  total  of  clear  and  fair  and  cloudy.  Repeat 


108 


WEATHER    ELEMENTS    AND    FORECASTING. 


this  process  of  summarizing  in  every  column.  Your  results 
will  then  show  the  percentages  of  the  different  kinds  of  weather 
noted  with  the  different  wind  directions. 

The  lower  division  of  the  table  and  the  last  two  columns  on 
the  right  are  to  be  used  for  a  general  summary  of  the  whole 
investigation.  By  adding  together  all  the  totals  of  clear,  fair, 
and  cloudy  weather  observed  with  all  the  eight  wind  directions 
you  obtain  the  whole  number  of  observations  you  have  made. 
Enter  this  in  the  space  marked  A,  at  the  right  of  the  table. 
From  this  grand  total  and  the  total  number  of  observations  in 
each  column  you  may  find  (in  percentages)  the  relative  frequen- 
cies of  the  different  wind  directions.  These  should  be  entered 
under  the  totals  at  the  bottom  of  each  column,  in  the  spaces 
marked  Percentages  (spaces  B,  C,  etc.).  The  total  number  of 
observations  of  clear,  fair,  cloud,  and  rain  or  snow,  noted  with 
all  the  wind  directions,  are  to  be  entered  in  spaces  D,  F,  G, 
and  H,  at  the  right  of  the  table.  From  these  totals,  and  from 
the  grand  total  in  space  A,  we  can  determine  the  relative 
frequency  (in  percentages)  of  each  kind  of  weather  during  the 
month.  These  results  should  be  entered  in  spaces  J,  K,  L, 
and  M. 

Study  these  results   carefully.     Formulate  them   in  a  brief 

written  statement.    Express  graph- 
ically the  following :  - 

A.  The  percentages  of  frequency 
of   the    different   wind   directions 
during  the  month. 

B.  The  percentages  of  the  differ- 
ent kinds  of  weather  noted  during 
the    whole    month    for    all    wind 
directions. 

A  wind  rose,  indicating  the 
percentages  of  frequency  of  dif- 
ferent winds  during  a  month,  or 


S.W. 


CYCLONES    AND    ANTICYCLONES.  109 

a  year,  or  several  years,  may  be  constructed  as  shown  in 
Fig.  51. 

A  certain  convenient  scale  is  adopted  as  representing  a  fre- 
quency of  10^>,  and  a  circle  is  drawn  with  this  unit  as  a  radius. 
A  second  circle,  with  a  radius  twice  as  long,  represents  a 
frequency  of  20/o,  and  a  third  circle,  with  a  radius  three  times 
as  long,  represents  a  frequency  of  30^.  Additional  circles  may 
be  added  if  necessary.  Distances  corresponding  to  the  differ- 
ent percentages  of  frequency  of  the  eight  wind  directions  are 
then  laid  off  along  the  eight  radii  of  the  circles,  and  the  points 
thus  fixed  are  joined  by  a  line. 

The  results  asked  for  under  question  B  may  be  plotted  as  a 
weather  rose  on  a  diagram  similar  to  that  above  figured.  In 
this  case  the  percentages  of  frequency  of  the  different  varieties 
of  weather  (clear,  fair,  cloudy,  stormy)  may  be  indicated  on  the 
same  figure  by  using  different  kinds  of  lines.  Thus,  a  solid  line 
may  be  employed  to  represent  clear  weather ;  a  broken  line  for 
fair;  a  broken  and  dotted  line  for  cloudy;  and  a  dotted  line  for 
stormy  weather. 


CHAPTER  XVI. 

CORRELATION   OF   CYCLONES  AND  ANTICYCLONES   AND   THE 
WEATHER. 

A.  Cyclones.  —  Prepare  a  piece  of  tracing  paper  as  shown  in 
Fig.  52,  making  the  diameter  of  the  outer  circle  about  1000 
miles 1  and  of  the  inner  circle  500  miles.  Place  this  diagram 
over  a  cyclone  on  any  weather  map,  centering  and  orienting  it 
carefully.  Trace  off  the  weather  signs  (indicating  clear,  fair, 
cloudy,  rain  or  snow)  around  the  cyclonic  center  from  the  map 
on  to  the  tracing  paper,  taking  only  observations  which  are  not 
more  than  halfway  from  the  cyclonic  center  to  the  neighbor- 
1  Use  the  scale  of  miles  given  on  the  weather  map. 


110  WEATHER    ELEMENTS    AND    FORECASTING. 

ing  anticyclonic  center.  Repeat  this  process  with  successive 
weather  maps  until  the  diagram  is  well  filled  in  all  its  different 
divisions. 

A.  Draw  a  line  on  the  tracing  paper  enclosing  the  average 
area  of  cloud  (including  rain  and  snow),  and  a  second  line  enclos- 
ing the  average  area  of  precipitation  (rain  or  snow). 

B.  Determine   the  percentages   of   clear,  fair,   cloudy,    and 

stormy  observations  for  each  divi- 
sion of  the  tracing  paper,  i.e.,  (a)  for 
the  eight  sectors  of  the  large  circle  ; 
(b)  for  the  whole  of  the  small 
circle;  and  (c)  for  the  portion  of 
the  diagram  between  the  circum- 
ference of  the  inner  circle  and  the 
circumference  of  the  outer  circle. 

0.    Write  out  in  general  terms 
a  description  of  the  weather  distri- 
bution in  cyclones  as  illustrated  by 
your  own  investigation. 
B.    Anticyclones.  —  This  exercise  is  done  in  the  same  way  as 

the  preceding  one,  except  that  anticyclones  are  substituted  for 

cyclones. 

The  association  of  fair  weather  with  anticyclones  and  of  stormy 
weather  with  cyclones  is  one  of  the  most  important  lessons  learned 
from  a  study  of  the  weather  maps.  The  great  areas  of  high  and  low 
pressure  control  our  weather.  On  land,  where  daily  weather  maps 
are  so  easily  accessible,  a  glance  at  the  map  serves  in  most 
cases  to  give  a  fairly  accurate  idea  of  the  position  and  extent  of 
cyclones  and  anticyclones,  and  hence  also  of  the  distribution  of 
weather.  At  sea,  on  the  other  hand,  the  navigator  has  no  daily 
weather  maps  to  refer  to,  and  his  knowledge  of  the  weather  condi- 
tions which  he  may  expect  must  be  gained  from  his  own  observa- 
tions alone.  Of  these  local  observations,  the  pressure  readings  are 
by  far  the  most  important.  A  falling  barometer  usually  means  the 
approach  of  a  cyclone,  with  wind,  or  storm,  or  both.  A  rising 


CYCLONES    AND    ANTICYCLONES.  HI 

barometer,  on  the  other  hand,  is  usually  an  indication  of  the  fine 
weather  associated  with  an  anticyclone.  The  unsymrnetrical  distri- 
bution of  weather,  characteristic  of  our  cyclones  in  the  United 
States,  and  also  of  most  cyclones  in  the  Temperate  Zone,  is  asso- 
ciated with  their  unsymmetrical  form,  and  the  unsymmetrical  dis- 
tribution of  their  temperature  already  studied.  Tropical  cyclones 
have  a  wonderfully  uniform  distribution  of  weather  on  all  sides  of 
their  centers,  just  as  they  have  a  symmetrical  form  and  an  even 
temperature  distribution  all  around  them. 


CHAPTER    XVII. 

PROGRESSION   OF   CYCLONES   AND   ANTICYCLONES. 

So  far  no  definite  study  has  been  made  of  the  changes  in  the 
positions  of  cyclones  and  anticyclones.  If  these  areas  of  stormy 
and  fair  weather  always  occupied  the  same  geographic  positions, 
the  different  portions  of  the  country  would  always  have  the 
same  kinds  of  weather.  A  knowledge  of  the  movements  of 
the  areas  of  low  and  high  pressure  makes  weather  forecasting 
possible. 

A.  Cyclones.  —  Select  a  set  of  daily  weather  maps  for  a  month. 
Turn  to  the  first  map  of  the  series.  Note  the  position  of  the 
center  of  low  pressure,  and  indicate  this  position  on  a  blank 
weather  map  of  the  United  States  by  marking  down  a  small 
circle  at  the  proper  place.  If  there  are  two  or  more  areas  of 
low  pressure  on  the  map,  indicate  the  position  of  each  one  of 
them  in  a  similar  way.  Turn  to  the  second  map  of  the  series, 
and  again  enter  on  the  blank  map  the  position  of  the  center  of 
low  pressure.  Connect  the  two  positions  of  each  center  by  a 
line.  This  line  may  be  called  the  track  of  the  low  pressure 
center.  Continue  this  process  through  the  whole  set  of  maps, 
connecting  all  the  new  positions  with  the  last  positions  of  their 
respective  centers.  Mark  each  position  with  the  appropriate 
date  in  small,  neat  figures.  When  completed  your  map  will 
show  at  a  glance  the  tracks  followed  by  all  the  cyclones  which 


112  WEATHER    ELEMENTS    AND    FORECASTING. 

traveled  across  the  United  States  during  the  month  you  selected. 
Study  these  tracks  carefully.  Notice  whether  there  is  any 
prevailing  direction  in  which  the  cyclones  move,  and  whether 
they  show  any  preference  for  particular  paths  across  the  country. 
Can  you  frame  a  general  rule  for  the  prevailing  direction  and 
path  of  movement  ?  Are  there  any  cases  which  do  not  accord 
with  the  rule  ?  If  so,  describe  them.  In  what  position,  with 
reference  to  the  cyclonic  tracks  during  the  month  you  are  study- 
ing, is  the  region  in  which  you  are  now  living? 

Next  determine  the  velocities  with  which  these  cyclones 
moved.  Prepare  a  scale  of  latitude  degrees,  as  described  in 
Chapter  V,  or  of  miles,  as  given  at  the  bottom  of  the  weather 
map.  Measure  the  distances,  in  miles,  between  the  successive 
positions  of  all  the  cyclonic  centers.  Divide  these  distances  by 
24  in  order  to  obtain  the  velocity  in  miles  per  hour.  What  is 
the  highest  velocity  per  hour  with  which  any  cyclone  moved 
during  the  month  ?  What  is  the  lowest  ?  What  is  the  mean, 
or  average,  velocity? 

Study  the  tracks  and  velocities  of  cyclones  in  a  similar  way 
during  several  other  months.  Compare  the  positions  of  the 
tracks,  and  the  velocities  of  progression,  in  summer  and  winter. 

B.  Anticyclones.  —  Study  the  tracks  and  velocities  of  anti- 
cyclones in  precisely  the  same  manner.  Compare  the  results 
derived  from  your  investigations  in  the  two  cases. 

Cyclonic  Tracks  and  the  Prevailing  Westerly  Winds.  —  The 
correspondence  in  the  direction  of  movement  of  most  of  our  cy- 
clones and  of  the  prevailing  westerly  winds  of  the  Northern  Hemis- 
phere (see  Chapter  IX)  will  readily  be  noted.  Our  weather  maps 
show  us  the  atmospheric  conditions  over  the  United  States  alone, 
and  we  can  therefore  trace  the  progression  of  our  cyclones  over 
but  a  limited  area  of  the  Northern  Hemisphere.  An  examina- 
tion of  the  daily  weather  maps  of  the  North  Atlantic  and  North 
Pacific  Oceans,  which  are  based  on  observations  made  on  board 
ships,  and  of  the  weather  maps  of  other  countries,  shows  us  that 
these  atmospheric  disturbances  which  appear  on  our  maps  may 


LOCAL    WEATHER    CHANGES. 


113 


often  be  traced  for  long  distances  across  oceans  and  lands,  and  that 
they  in  reality  form  a'  great  procession  across  the  northern  portion 
of  this  hemisphere,  and  towards  and  around  the  North  Pole. 

The  average  velocity  of  cyclones  in  the  United  States  was  care- 
fully determined  by  Loomis.  His  observations  show  that  the  mean 
hourly  velocity  of  cyclones  for  the  entire  year  is  28.4  miles,  the 
maximum  (34.2  miles)  coming  in  February,  and  the  minimum 
(22,6  miles)  in  August.  Over  the  North  Atlantic  Ocean  the  hourly 
velocity  is  18-  miles ;  in  Europe,  16.7  miles.  The  greater  velocity 
of  cyclonic  movement  in  the  United  States  in  winter  recalls  what 
was  said  at  the  close  of  Chapter  X  concerning  the  steeper  baro- 
metric gradient  and  the  more  rapid  movement  of  the  whole  atmos- 
phere over  the  Northern  Hemisphere  in  winter. 


CHAPTER    XVIII. 

SEQUENCE   OF  LOCAL  WEATHER   CHANGES. 

THE  next,  and  last,  step  in  our  study  of  the  correlation  of 
the  various  weather  elements  concerns  the  sequence  of  weather 
changes  at  a  station  before,  during,  and  after  the  passage  of  a 
cyclone  and  of  an  anticyclone. 

A.  Cyclones.  —  I.  Select  some  station  which  the  weather 
maps  show  to  have  been  directly  on  the  track  of  a  well-devel- 
oped cyclone,  i.e.,  to  have  been  passed  over  by  the  center  of 
the  cyclone.  Note  the  weather  conditions  at  this  station, 
before,  during,  and  after  the  passage  of  the  storm.  Tabulate 
your  observations  according  to  the  following  scheme  :  — 

TABLE  V. 

Weather  Changes  at during 


DATES 

PRESSURE 

TEMPER- 
ATURE „ 

WIND 
DIRECTION 

WEND 
VELOCITY 

WEATHER 

DlREC.  AND  DlST. 

OF  STORM  CENTER 

114  WEATHER    ELEMENTS    AND    FORECASTING. 

In  the  last  column  of  the  table  enter  the  direction  and  the 
distance  of  the  cyclonic  center  from  the  station,  at  each  obser- 
vation. 

II.  Select  a  station  which  was  north  of  the  track  of  a  cy- 
clone, and  tabulate  (in  a  separate  table)  the  weather  conditions 
at   that   station   before,  during,  and  after  the  passage   of  the 
center. 

III.  Do  the  same  for  a  station  which  was  south  of  the  track 
of  a  cyclone.     Repeat  these  observations  for  several  stations. 

B.  Anticyclones.  —  Make  a  similar  series  of  observations  for 
the  passage  of  an  anticyclone  centrally  over,  north  and  south 
of,  several  stations. 

Study  the  sequence  of  the  weather  changes  shown  in  the 
various  tables.  Deduce  a  general  rule  for  these  changes  and 
write  it  out. 


CHAPTER    XIX. 

WEATHER   FORECASTING. 

Ix  a  letter  dated  at  Philadelphia,  July  16,  174T,  Benjamin 
Franklin  wrote  to  his  friend  Jared  Eliot  as  follows  :  "  We  have 
frequently  along  the  North  American  coast  storms  from  the 
northeast  which  blow  violently  sometimes  three  or  four  days. 
Of  these  I  have  had  a  very  singular  opinion  for  some  years, 
viz.:  that,  though  the  course  of  the  wind  is  from  northeast  to 
southwest,  yet  the  course  of  the  storm  is  from  southwest  to 
northeast;  the  air  is  in  violent  motion  in  Virginia  before  it 
moves  in  Connecticut,  and  in  Connecticut  before  it  moves 
at  Cape  Sable,  etc.  My  reason  for  this  opinion  (if  the  like 
have  not  occurred  to  you)  I  will  give  in  my  next." 

In  a  second  letter  to  the  same  correspondent,  dated  Phila- 
delphia, Feb.  13,  1749-50,  Franklin  states  his  reasons  as  fol- 
lows :  "  You  desire  to  know  my  thoughts  about  the  northeast 


WEATHER    FORECASTING.  115 

storms  beginning  to  leeward.  Some  years  since,  there  was  an 
eclipse  of  the  moon  at  nine  o'clock  in  the  evening,  which  I 
intended  to  observe ;  but  before  night  a  storm  blew  up  at  north- 
east, and  continued  violent  all  night  and  all  the  next  day ;  the 
sky  thick-clouded,  dark,  and  rainy,  so  that  neither  moon  nor 
stars  could  be  seen.  The  storm  did  great  damage  all  along  the 
coast,  for  we  had  accounts  of  it  in  the  newspapers  from  Boston, 
Newport,  New  York,  Maryland,  and  Virginia ;  but  what  surprised 
us  was  to  find  in  the  Boston  newspapers  an  account  of  the 
observation  of  that  eclipse  made  there;  for  I  thought  as  the 
storm  came  from  the  northeast  it  must  have  begun  sooner  at 
Boston  than  with  us,  and  consequently  have  prevented  such  an 
observation.  I  wrote  to  my  brother  about  it,  and  he  informed 
me  that  the  eclipse  was  over  there  an  hour  before  the  storm 
began.  Since  which  time  I  have  made  inquiries  from  time  to 
time  of  travelers,  and  observed  the  accounts  in  the  newspapers 
from  New  England,  New  York,  Maryland,  Virginia,  and  South 
Carolina;  and  I  find  it  to  be  a  constant  fact  that  northeast 
storms  begin  to  leeward,  and  are  often  more  violent  there  than 
to  windward"  (Sparks's  Life  of  Franklin,  VI,  79,  105,  106). 

The  fact  that  our  northeast  storms  come  from  the  southwest, 
which  was  first  noticed  by  Benjamin  Franklin  some  years  before 
he  put  the  suggestion  just  quoted  in  writing,  was  one  of  the 
great  contributions  to  meteorology  made  by  Americans.  Mod- 
ern weather  forecasting  essentially  depends  upon  the  general 
eastward  movement  of  cyclones  and  anticyclones,  with  their 
accompanying  weather  conditions. 

The  daily  weather  map  shows  us  the  actual  condition  of  the 
weather  all  over  the  United  States  at  8  A.M.,  "  Eastern  Standard 
Time."  The  positions  of  cyclones  and  of  anticyclones  ;  of  areas 
of  clear,  fair,  cloudy  or  stormy  weather,  and  of  regions  of  high 
or  low  temperatures,  are  plainly  seen  at  a  glance.  These  areas 
of  fair  and  foul  weather,  with  their  accompanying  systems  of 
spiralling  winds,  move  across  country  in  a  general  easterly 


116  WEATHER    ELEMENTS    AND    FORECASTING. 

direction.  Knowing  something  of  their  direction  and  rate  of 
movement,  we  can  determine,  with  greater  or  less  accuracy, 
their  probable  positions  in  12,  24,  36,  or  48  hours.  The  predic- 
tion or  foretelling  of  the  weather  which  may  be  expected  to 
prevail  at  any  station  or  in  any  district  is  weather  forecasting . 

Weather  forecasts  are  usually  made  on  our  daily  weather 
maps  for  24  hours  in  advance.  It  is  by  110  means  an  easy 
thing  to  make  accurate  weather  forecasts.  Careful  study  and 
much  practice  are  required  of  the  forecasters  of  the  Weather 
Bureau  before  they  are  permitted  to  make  the  official  forecasts 
which  are  printed  on  the  daily  maps  and  in  the  newspapers. 

A  simple  extension  and  application  of  the  principles  learned 
through  the  preceding  exercises  make  it  possible  for  us  to 
forecast  coming  weather  changes  in  a  general  way.  These 
suggestions  are,  however,  not  at  all  to  be  considered  as  a 
complete  discussion  of  this  complicated  problem. 

Weather  forecasts  include  the  probable  changes  in  tempera- 
ture, wind  direction  and  velocity,  and  weather.  Pressure  is  not 
included.  Begin  your  practice  in  weather  forecasting  by  con- 
sidering only  the  changes  that  may  be  expected  at  your  own 
point  of  observation,  and  at  first  confine  yourself  to  predicting 
temperature  changes  alone. 

Temperature.  —  Provide  yourself  with  a  blank  weather  map. 
Draw  an  isotherm  east  and  west  across  the  map,  through  your 
station.  Draw  a  few  other  isotherms  all  the  way  across  the 
map,  parallel  with  the  first  one,  and  so  arranged  that  they  will 
be  equal  distances  apart,  the  most  northerly  one  running  through 
northern  Maine  and  the  Northwestern  States,  and  the  most 
southerly  one  through  southern  Florida  and  Texas.  Recalling 
what  you  have  already  discovered  concerning  the  eastward 
movement  of  our  weather  conditions,  what  forecast  will  you 
make  as  to  the  coming  temperatures  at  your  station?  Add 
some  additional  east  and  west  isotherms,  so  that  there  will  be 
twice  as  many  on  your  map  as  before.  What  effect  will  this 


WEATHER    FORECASTING.  117 

change  in  the  temperature  distribution  on  your  map  have  upon 
the  temperature  forecast  you  make  for  your  station  ?  Formu- 
late a  general  rule  as  to  temperature  forecasts  under  the  condi- 
tions of  isothermal  arrangement  here  suggested. 

On  a  second  blank  weather  map  draw  an  isotherm  through 
your  station  inclined  from  northwest  to  southeast.  Draw  a  few 
other  isotherms  parallel  to  the  first,  and  each  one  representing 
a  temperature  10°  higher  than  that  indicated  by  the  adjacent 
isotherm  on  the  east.  Make  a  general  forecast  of  the  tempera- 
ture conditions  that  may  be  expected  at  your  station,  as  to  kind 
of  change,  if  any ;  amount  of  change,  and  rapidity  of  change.  Of 
the  isotherms  just  drawn,  erase  every  second  one ;  still,  however, 
letting  those  that  are  left  represent  differences  of  temperature 
of  10°.  What  forecast  will  you  now  make  as  to  temperature? 
How  does  this  forecast  compare  with  that  just  made  ? 

Now  draw  twice  as  many  isotherms  on  your  map  as  you  had 
in  the  first  place,  still  letting  these  lines  represent  differences 
of  temperature  of  10°  in  each  case.  Make  a  forecast  of  the 
kind,  amount,  and  rapidity  of  temperature  change  at  your 
station  under  the  conditions  represented  on  this  map.  How 
does  this  forecast  compare  with  the  two  just  made  ?  Formulate 
a  general  rule  governing  temperature  forecasts  in  cases  of  iso- 
thermal arrangement  such  as  those  here  considered. 

Take  another  blank  map.  Draw  through  your  station  an 
isotherm  inclined  from  northeast  to  southwest.  Draw  other 
isotherms  parallel  to  this,  west  of  your  station,  letting  each 
successive  isotherm  represent  a  temperature  10°  lower  than  that 
indicated  by  the  adjacent  isotherm  on  the  east.  Make  a  tempera- 
ture forecast  for  your  station  under  these  conditions.  Diminish 
and  increase  the  number  of  isotherms  on  your  map,  as  suggested 
in  the  preceding  example,  making  temperature  forecasts  in  each 
case,  and  comparing  the  three  sets  of  forecasts.  Formulate  a 
general  rule  for  temperature  forecasts  made  under  these  systems 
of  isotherms. 


118  WEATHEK    ELEMENTS    AND    FORECASTING. 

Make  temperature  forecasts  from  the  daily  weather  maps  for 
your  own  station,  using  the  knowledge  that  you  have  already 
gained  as  to  the  progression  of  cyclones  and  anticyclones 
(Chapter  XVII),  and  as  to  the  temperature  distribution  in 
these  areas  (Chapter  XIV),  to  help  you  in  this  work.  Study 
each  day's  map  carefully  before  you  decide  on  what  you  will 
say.  Then  write  out  your  own  forecast,  and  afterwards  com- 
pare  your  forecast  with  that  made  by  the  Weather  Bureau. 
Note  also,  by  reference  to  your  own  instrumental  observations, 
whether  the  succeeding  temperature  conditions  are  such  as  you 
predicted. 

Wind  Direction, — The  weather  maps  already  studied  taught 
us  that  our  winds  habitually  move  in  spirals.  The  composite 
picture  of  the  wind  circulation  around  cyclones  and  anticyclones 
(Chapter  XII)  further  emphasized  this  important  fact.  Evi- 
dently this  law  of  the  systematic  circulation  of  the  winds  around 
centers  of  low  and  high  pressure  may  be  utilized  in  making 
forecasts  of  wind  direction. 

Applying  the  knowledge  already  gained  concerning  cyclonic 
and  anticyclonic  wind  circulations,  ask  yourself  what  winds  a 
station  should  have  which  is  within  the  range  of  the  cyclonic 
wind  system,  and  is  in  the  following  positions  with  reference  to 
the  center:  northeast,  north,  northwest,  east,  at  the  center,  west, 
southeast,  south,  southwest.  Ask  yourself  precisely  the  same 
questions  with  reference  to  a  station  within  an  anticyclonic 
wind  system.  Write  out  a  general  rule  for  the  kinds  of  wind 
changes  which  may  be  expected  to  take  place  under  these 
different  conditions. 

When  a  station  is  south  of  the  track  of  a  passing  cyclone  its 
winds  are  said  to  veer,  and 'the  change  in  the  direction  of  its 
winds  is  called  veering.  A  station  north  of  the  track  of  a 
passing  cyclone  has  a  change  of  direction  in  its  winds  which 
is  known  as  backing,  the  winds  themselves  being  said  to  back. 

Wind  Velocity.  —  What  general  relation  between  wind  veloci- 


WEATHER    FORECASTING.  119 

ties  and  areas  of  low  and  high  pressure  did  you  discover  in  your 
study  of  the  weather  maps?  What  was  the  result  of  your 
work  on  the  correlation  of  the  velocity  of  the  wind  and  the 
barometric  gradient  in  Chapter  X?  And  what  general  state- 
ment as  to  the  relation  between  the  velocity  of  the  wind  and  its 
distance  from  a  cyclonic  or  anticyclonic  center  may  be  made  as 
the  result  of  your  work  in  Chapter  XII,  on  the  correlation  of 
cyclones  and  anticyclones  with  their  wind  circulation  ?  These 
results  must  be  borne  in  mind  in  making  predictions  of  coming 
changes  in  wind  velocities.  Forecasts  of  wind  velocities  are 
made  in  general  terms  only,  —  light,  moderate,  fresh,  brisk,  high, 
gale,  hurricane,  —  and  are  not  given  in  miles  per  hour. 

Make  forecasts  of  wind  direction  and  velocity  from  the  daily 
weather  maps  for  your  own  station.  Continue  these  for  a  week 
or  two,  keeping  record  of  the  verification  or  non-verification  of 
each  of  your  forecasts.  Then  make  daily  forecasts  of  tempera- 
ture and  of  wind  direction  and  velocity  together.  Write  out 
your  own  forecast  for  each  day  before  you  compare  it  with  the 
official  forecast,  and  if  the  two  differ,  keep  note  of  which  one 
seemed  to  you  to  be  the  most  accurate. 

Weather.  —  What  general  relation  between  kind  of  weather 
and  cyclones  and  anticyclones  was  illustrated  on  the  six  maps 
of  our  series?  What  is  the  average  distribution  of  the  dif- 
ferent kinds  of  weather  around  cyclones  and  anticyclones,  as 
shown  by  your  composites  ?  (Chapter  XVI.)  What  changes 
in  weather  will  ordinarily  be  experienced  at  a  station  as  a 
cyclone  approaches,  passes  over,  and  moves  off  ?  What  condi- 
tions will  prevail  in  an  anticyclone  ? 

Make  a  series  of  daily  forecasts  for  your  own  station  of 
probable  weather  changes,  omitting  temperature  and  winds  at 
first.  Include  in  your  weather  forecasts  the  state  of  the  sky 
(clear,  fair,  cloudy)  ;  the  changes  in  the  state  of  the  sky 
(increasing  or  decreasing  cloudiness) ;  the  kind  of  precipitation 
(rain  or  mow)  and  the  amount  of  precipitation  (light  or  heavy). 


120 


WEATHER    ELEMENTS    AND    FORECASTING. 


Write  out  your  forecasts ;  compare  them  with  the  official  fore- 
casts, and  notice  how  fully  they  are  verified.  Then  add  tem- 
perature and  winds  to  your  forecasts  so  that  you  will  make  a 
complete  prediction  of  probable  changes  in  temperature  (kind, 
amount,  and  rapidity  of  change),  wind  (direction  and  velocity), 
and  weather.  Practice  making  these  complete  forecasts  for 
several  weeks,  if  time  allows.  Use  all  the  knowledge  that 


FIG.  53. 

you  have  gained  in  the  preceding  work  to  aid  you  in  this. 
Study  each  weather  map  very  carefully.  Do  not  write  down 
your  forecast  until  you  are  sure  that  you  have  done  the  best 
you  can. 

Vary  this  exercise  by  extending  your  forecasts  so  as  to 
embrace  the  whole  section  of  country  in  which  your  station 
is  situated  (as,  e.g.,  New  England,  the  Gulf  States,  the  Lake 
region).  Pay  special  attention  to  making  forecasts  of  cold 


WEATHER    FORECASTING.  121 

waves,  of  heavy  rain  or  snowstorms,  of  high  winds  over  the 
lakes  or  along  the  Atlantic  coast,  etc.  When  possible,  obtain 
from  the  daily  newspapers  any  particulars  as  to  damage  done 
by  frost  or  gales,  or  concerning  snow  blockades,  floods  due  to 
heavy  rains,  etc. 

Fig.  53  summarizes  what  has  thus  far  been  learned  as  to  the 
distribution  of  the  various  weather  elements  around  a  well- 
developed  center  of  low  pressure.  The  curved  broken  lines 
represent  the  isotherms  (Chapter  XIV).  The  solid  concentric 
oval  lines  are  the  isobars  (Chapter  XI).  The  arrows  repre- 
sent the  winds,  the  lengths  of  the  arrows  being  roughly  pro- 
portionate to  the  wind  velocities  (Chapter  XII).  The  whole 
shaded  area  represents  the  region  over  which  the  sky  is  covered 
by  heavy  lower  clouds.  The  smaller  shaded  area,  within  the 
larger,  encloses  the  district  over  which  rain  or  snow  is  falling 
(Chapter  XVI).  The  lines  running  out  in  front  of  the  cloudy 
area  represent  the  light  upper  clouds  (cirrus  and  cirro-stratus) 
which  usually  precede  an  area  of  low  pressure. 

Imagine  this  whole  disturbance  moving  across  the  United 
States  in  a  northeasterly  direction,  and  imagine  yourself  at  a 
station  (1)  directly  in  the  path  of  the  cyclone  ;  (2)  south  of  the 
track ;  and  (3)  north  of  the  track.  In  the  first  case,  as  the  dis- 
turbance moved  on  in  its  path,  you  would  successively  occupy 
the  positions  marked  A,  B,  and  C  on  the  line  AC,  passing 
through  the  center  of  the  cyclone.  In  the  second  case  you 
would  be  first  at  D,  then  at  E,  and  then  at  F.  In  the  third 
case  you  would  be  at  Gr,  H,  and  «7in  succession.  What  changes 
of  weather  would  you  experience  in  each  of  these  positions  as1 
the  cyclone  passed  by  you  ?  Imagine  yourself  at  some  station 
halfway  between  the  lines  AC  arid  DF.  What  weather  changes 
would  you  have  in  that  position  with  reference  to  the  storm 
track?  In  what  respects  would  these  weather  changes  differ 
from  those  experienced  along  the  line  DF?  Imagine  your 
station  halfway  between  the  lines  AC  and  GrJ.  What  weather 


122  WEATHER    ELEMENTS    AND    FORECASTING. 

changes  would  you  have  there?  How  would  these  changes 
differ  from  those  experienced  along  the  line  GJ? 

It  must  be  remembered  that  Fig.  53  is  an  ideal  diagram.  It 
represents  conditions  which  are  not  to  be  expected  in  every 
cyclone  which  appears  on  our  weather  maps.  If  all  cyclones 
were  exactly  alike  in  the  weather  conditions  around  them, 
weather  forecasting  would  be  a  very  easy  task.  But  cyclones 
are  not  all  alike  —  far  from  it.  Some  are  well  developed,  with 
strong  gradients,  high  winds,  extended  cloud  areas,  heavy  pre- 
cipitation, and  decided  temperature  contrasts.  Others  are  but 
poorly  developed,  with  weak  gradients,  light  winds,  small  tem- 
perature differences,  and  it  may  be  without  any  precipitation, 
whatever.  Some  cover  immense  districts  of  country;  others 
are  small  and  affect  only  a  limited  area.  It  therefore  becomes 
necessary  to  examine  the  characteristics  of  each  approaching 
cyclone,  as  shown  on  the  daily  weather  map,  very  carefully. 
Notice  whether  it  is  accompanied  by  heavy  rain  or  snow; 
whether  its  winds  are  violent ;  how  far  ahead  of  the  center 
the  cloudy  area  extends  ;  how  far  behind  the  outer  cloud  limit 
the  rain  area  begins  ;  what  is  the  position  of  the  cloud  and  rain 
area  with  reference  to  the  center,  and  other  points  of  equal 
importance,  and  govern  yourself,  in  making  your  forecast, 
according  to  the  special  features  of  each  individual  cyclone. 
Well-developed  cyclones  will  be  accompanied  by  marked 
weather  changes.  Weak  cyclones  will  have  their  weather 
changes  but  faintly  marked. 

The  distance  of  your  station  from  the  center  of  the  cyclone 
is  of  great  importance  in  determining  what  the  weather  con- 
ditions and  changes  shall  be,  as  may  easily  be  seen  by  examin- 
ing Fig.  53.  If  the  storm  passes  far  to  the  north  or  far  to  the 
south  of  your  station,  you  may  notice  none  of  its  accompany- 
ing weather  conditions,  except,  perhaps,  a  bank  of  clouds  on 
your  horizon.  You  may  for  a  few  hours  be  under  the  cloudy 
sky  of  some  passing  storm,  and  yet  not  be  reached  by  its  rainy 


WEATHER    FORECASTING.  123 

area.  The  shifts  in  the  wind  may  be  marked  and  the  wind 
velocities  high,  or  the  expected  veering  or  backing  may  hardly 
be  noticeable,  owing  to  the  weakness  or  the  distance  of  the 
controlling  cyclone. 

Again,  the  rapidity  with  which  weather  conditions  will 
change  depends  upon  the  rate  of  movement  of  the  cyclone 
itself.  The  better  developed  the  cyclone,  the  higher  its 
velocity  of  progression,  and  the  nearer  its  track  lies  to  the 
station,  the  more  emphatic  and  the  more  rapid  are  the  weather 
changes  it  causes.  On  the  other  hand,  the  weaker  the  cyclone, 
the  slower  its  rate  of  progression,  and  the  further  away  its 
track,  the  less  marked  and  the  slower  the  weather  changes. 
The  probable  track  of  a  coming  storm,  and  its  probable  rate 
of  movement,  therefore,  need  careful  study  if  our  forecasts  are 
to  be  reliable. 

There  are  many  other  obstacles  in  the  way  which  combine 
to  render  weather  forecasting  extremely  difficult.  Some  of 
these  difficulties  you  will  learn  to  overcome  more  or  less  suc- 
cessfully by  the  experience  you  will  gain  from  a  careful  and 
persevering  study  of  the  daily  weather  maps  ;  others,  the  best 
forecast  officials  of  our  Weather  Bureau  have  not  yet  entirely 
overcome.  The  tracks  followed  by  our  cyclones  vary  more  or 
less  from  month  to  month,  and  even  if  the  average  tracks 
for  each  month  are  known,  individual  cyclones  may  occur 
which  absolutely  disregard  these  tracks.  While  the  average 
hourly  velocity  of  cyclones  is  accurately  known  for  the  year 
and  for  each  month,  the  movements  of  individual  storms  are 
often  very  capricious.  They  may  move  with  a  fairly  uni- 
form velocity  throughout  the  time  of  their  duration ;  they 
may  suddenly  and  unexpectedly  increase  their  rate  of  move- 
ment, or  they  may  as  suddenly  come  nearly  to  a  standstill. 
The  characteristics  of  cyclones  vary  in  different  portions  of  the 
country  and  at  different  times.  Cyclones  which  have  been 
accompanied  by  little  precipitation  on  most  of  their  journey 


124  WEATHER    ELEMENTS    AND    FORECASTING. 

are  apt  to  give  increased  rain  or  snowfall  as  they  near  the 
Atlantic  Ocean  and  Gulf  of  St.  Lawrence.  Cyclones  which 
over  one  portion  of  the  country  were  rainy,  may  give  little 
or  no  precipitation  in  another  portion.  Cyclones  and  anticy- 
clones are  found  to  have  considerable  influence  on  one  another, 
retarding  or  accelerating  one  another's  advance,  or  changing 
one  another's  normal  path  of  progression.  While  this  mutual 
interaction  is  clearly  seen,  and  may  be  successfully  predicted  in 
many  cases,  many  other  cases  arise  in  which,  under  apparently 
similar  conditions,  the  result  is  very  different  from  the  antici- 
pation. Such  are  some  of  the  difficulties  with  which  weather 
forecasters  have  to  contend,  and  which  prevent  the  attainment 
of  greater  accuracy  in  weather  prediction. 


PART   V.  —  PROBLEMS   IN   OBSERVATIONAL 
METEOROLOGY. 


CHAPTER   XX. 

S'; 

TEMPERATURE . 

THE  chief  interest  and  value  of  the  instrumental  work 
in  meteorology  are  to  be  found  not  only  in  the  taking  of  the 
daily  observations  at  stated  hours,  but  in  the  working  out  of 
numerous  simple  problems,  such  as  may  readily  be  undertaken 
with  the  help  of  the  instruments  already  described.  Thus,  the 
temperature  of  the  air  (obtained  by  the  sling  thermometer, 
supplemented  by  maximum  and  minimum  thermometers,  and 
by  the  thermograph  if  available)  can  be  determined  under  a 
variety  of  conditions,  e.g.,  close  to  the  ground,  and  at  different 
heights  above  the  ground ;  at  different  hours,  by  day  and  night ; 
in  different  seasons;  in  sunshine  and  in  shade;  during  wind 
and  calms ;  in  clear  and  cloudy  weather ;  in  woods  and  in  the 
open;  over  bare  ground,  grass,  snow,  or  ice;  on  hills  and  in 
valleys.  Observations  may  also  be  made  of  the  temperature  of 
the  ground  and  of  a  snow  cover,  at  the  surface  and  at  slight 
depths  beneath  the  surface,  in  different  seasons  and  under  dif- 
ferent weather  conditions.  Among  the  problems  which  may 
be  worked  out  by  means  of  such  observations  as  these  are  the 
following :  — 

A.  The  Diurnal  Range  of  Temperature  under  Different  Conditions 
and  at  Different  Heights  above  the  Ground.  —  Under  the  influence 
of  the  sun  the  regular  normal  variation  of  temperature  during 

125 


126  OBSERVATIONAL  METEOROLOGY. 

24  hours  is  as  follows:  A  gradual  increase,  with  the  increas- 
ing altitude  of  the  sun,  from  sunrise  until  shortly  after  noon, 
and  a  gradual  decrease,  with  decreasing  altitude  of  the  sun, 
from  the  maximum,  shortly  after  noon,  until  the  minimum, 
about  sunrise.  This  variation  is  known  as  the  diurnal  varia- 
tion of  temperature.  Curve  a  in  Fig.  12  illustrates  well  the 
normal  diurnal  variation  of  temperature,  as  recorded  by  the 
thermograph  during  a  period  of  clear,  warm  spring  weather 
(April  27-30,  1889,  Nashua,  N.  H.).  The  diurnal  range  of 
temperature  is  the  difference  between  the  maximum  and  mini- 
mum of  the  diurnal  oscillation.  The  regular  normal  diurnal 
variation  in  temperature  is  often  much  interfered  with  by 
other  controlling  causes  than  the  sun,  e.g.,  cyclonic  winds, 
clouds,  etc. 

I.  Study  and  compare  the  diurnal  ranges  of  temperature  as 
indicated  by  the  maximum  and  minimum  thermometers,  or  the 
thermograph,  in  the  instrument  shelter,  in  clear,  fair,  cloud}% 
and  stormy  weather,  during  winds  and  in  calms,  in  different 
months.     Summarize  your  results  by  grouping  them  according 
to  the  general  weather  conditions,  and  according  to  the  months 
or  seasons  in  which  the  observations  were  made.     For  example, 
group  together  and  average  the  ranges  observed  on  clear,  calm 
days  in  winter ;  on  similar  days  in  early  summer  or  autumn ; 
on  clear  days  with  brisk  northwest  winds  in  winter;  on  similar 
days  in  early  summer  or  autumn ;  on  calm  days  with  overcast 
sky  in  the  different  seasons ;  on  stormy  days  with  strong  winds, 
etc.     Study  carefully  the  weather  maps  for  the  days  on  which 
your   observations    are    made.     Pay   special    attention   to  the 
relation  between  the  diurnal  ranges  and  the  control  exercised 
over  these  ranges  by  cyclones  and   anticyclones  through  their 
winds  and  general  weather  conditions. 

II.  Observations  of  diurnal  ranges  of  temperature  at  differ- 
ent heights  above  the  ground  may  be  made  by  means  of  maxi- 
mum and  minimum  thermometers  fastened  (temporarily)  outside 


TEMPERATURE.  127 

of  the  windows  of  different  stories  of  the  school  or  of  some 
other  building.  These  observations  should  be  made  out  of 
windows  facing  north,  and  care  should  be  taken  to  check,  so  far 
as  possible,  any  draft  from  within  the  building  out  through  the 
window  during  the  taking  of  the  observation.  If  a  fire  escape 
is  provided  on  the  building,  the  instruments  may  often  be 
conveniently  fastened  to  that. 

Study  the  ranges  under  different  conditions  of  wind  and 
weather  at  various  heights  above  the  ground,  and  compare 
these  results  with  those  obtained  under  I.  Notice  the  relations 
of  all  your  results  to  the  cyclonic  and  anticyclonic  areas  of  the 
weather  maps. 

The  diurnal  range  of  temperature  in  the  air  over  the  open  ocean 
from  the  equator  to  latitude  40°  has  been  found  to  average  only 
2°  to  3°.  In  southeastern  California  and  the  adjacent  portion  of 
Arizona  the  average  diurnal-  temperature  range  in  summer  is  40° 
or  45°.  Over  other  arid  regions,  such  as  the  Sahara,  Arabia,  and 
the  interior  of  Australia,  the  range  also  often  amounts  to  40°. 
Observations  of  temperature  above  the  earth's  surface,  in  the  free 
air,  made  on  mountains,  in  balloons,  and  by  means  of  instruments 
elevated  by  kites,  indicate  very  clearly  that  the  diurnal  range  of 
temperature  decreases  with  increasing  elevation  above  sea  level. 
The  results  obtained  at  Blue  Hill  Observatory,  Massachusetts,  by 
means  of  kites,  show  that  the  diurnal  range  of  temperature  almost 
disappears,  on  the  average,  at  3300  feet  (1000  meters). 

B.  Changes  of  Temperature  in  the  Lower  Air,  and  their  Control 
by  the  Condition  of  the  Ground,  the  Movement  of  the  Air,  and  Other 

Factors, Determine  the  changes  of  the  temperature  in  the  lower 

air  by  making  frequent  readings  of  the  ordinary  thermometer  in 
the  instrument  shelter,  of  the  sling  thermometer,  or  by  an  exam- 
ination of  the  thermograph  record.  Group  these  changes,  as  in 
Problem  A,  so  far  as  possible  according  to  the  weather  con- 
ditions under  which  they  occurred,  and  try  to  classify  the  kinds 
of  change  roughly  into  types.  Study  the  control  of  these 


128  OBSERVATIONAL  METEOROLOGY. 

various  types  by  the  wind  and  other  weather  conditions  accom- 
panying them,  as  illustrated  on  the  daily  weather  maps.  The 
control  exercised  by  different  conditions  of  the  earth's  surface 
may  be  studied  by  means  of  observations  made  with  the  sling 
thermometer  over  different  surfaces,  such  as  grass,  bare  ground, 
snow,  etc. 

Examples  of  temperature  changes  in  the  lower  air,  under 
different  conditions  of  weather,  recorded  on  the  thermograph, 
are  given  in  Fig.  12,  and  are  briefly  referred  to  their  causes  in 
the  text  accompanying  that  figure. 

C.  Vertical  Distribution  of  Temperature  in  the  Atmosphere-  — 
The  vertical  distribution  of  temperature  in  the  lower  air  may 
be  studied  by  having  ordinary  thermometers  or  thermographs 
exposed  at  different  heights  above  the  ground,  e.g.,  close  to  the 
surface  ;  in  an  instrument  shelter ;  out  of  windows  on  succes- 
sive stories  of  some  high  building;  and  on  the  roof  of  the 
building.  They  may  also,  in  cases  where  there  is  a  hill  in  the 
neighborhood,  be  exposed  in  a  valley  at  the  bottom  of  the  hill 
and  at  successive  elevations  up  the  side  of  the  hill.  It  is,  how- 
ever, usually  much  simpler,  as  well  as  more  practicable,  to  take 
these  temperature  readings  by  means  of  the  sling  thermometer. 
In  the  case  of  observations  made  out  of  the  windows  of  a  build- 
ing, one  observer  can  take  the  readings  at  different  elevations 
in  succession.  When  the  observations  are  made  at  different 
altitudes  on  the  side  of  a  hill,  it  is  best  to  have  the  cooperation 
of  several  observers,  who  shall  all  read  their  thermometers  at 
the  same  moment  of  time.  The  results  obtained  in  the  previous 
problems  ( A  and  B)  may,  of  course,  also  be  utilized  in  studying 
the  vertical  distribution  of  temperature  in  the  atmosphere. 

Study  the  vertical  distribution  of  temperature  in  the  lower 
air  under  various  conditions  of  weather  and  season ;  at  various 
hours  of  the  day,  and  with  varying  conditions  of  surface  cover. 
Make  your  observations  systematically,  at  regular  hours,  so  that 
the  results  may  be  comparable.  Group  together  observations 


TEMPERATURE.  129 

made  under  similar  conditions  of,  weather,  season,  time,  and 
surface  cover.  Determine  the  average  vertical  distribution  of 
temperature  in  the  different  cases.  Note  especially  any  seem- 
ing peculiarities  or  irregularities  in  this  distribution  at  certain 
times.  Study  carefully,  as  in  the  previous  problems,  the  relation 
of  the  different  types  of  temperature  distribution  in  the  atmos- 
phere to  the  weather  conditions  as  shown  on  the  daily  weather 
maps. 

Observations  made  in  different  parts  of  the  world,  on  mountains 
and  in  balloons,  have  shown  that  on  the  average  the  temperature 
decreases  from  the  earth's  surface  upwards  at  the  rate  of  about  1°  in 
300  feet  of  ascent.  The  rate  of  vertical  decrease  of  temperature  is 
known  in  meteorology  as  the  vertical  temperature  gradient.  When  it 
happens  that  there  is  for  a  time  an  increase  in  temperature  upwards 
from  the  earth's  surface,  the  condition  is  known  as  an  inversion  of 
temperature. 

As  a  result  of  the  decrease  of  temperature  with  increasing  alti- 
tude above  sea  level,  the  tops  of  many  high  mountains  even  in  the 
Torrid  Zone  are  always  covered  with  snow,  while  no  snow  can  ever 
fall  at  their  bases,  owing  to  the  high  temperatures  which  prevail 
there.  Balloons  sent  up  without  aeronauts,  but  with  self-recording 
instruments,  have  given  us  temperatures  of  —  90°  at  a  height  of 
10  miles  above  the  earth's  surface.  On  Dec.  4,  1896,  Berson 
reached  a  height  of  30,000  feet  and  noted  a  temperature  of  —  52°. 
Inversions  of  temperature  are  quite  common,  especially  during  the 
clear  cold  spells  of  winter.  Under  such  conditions  the  tops  and 
sides  of  hills  and  mountains  are  often  much  warmer  than  the 
valley  bottoms  at  their  bases.  A  good  example  of  an  inversion  of 
temperature  occurred  in  New  Hampshire  on  Dec.  27,  1884.  The 
pressure  was  above  the  normal,  the  sky  clear  and  the  wind  light. 
The  observer  on  the  summit  of  Mt.  Washington  reported  a  tempera- 
ture of  -f  16°  on  the  morning  of  that  day,  while  the  thermometers 
on  the  neighboring  lowlands  gave  readings  of  from  —10°  to  —24°. 
In  Switzerland,  the  villages  and  cottages  are  generally  built  on  the 
mountain  sides  and  not  down  in  the  valley  bottoms,  experience 
having  taught  the  natives  that  the  greatest  cold  is  found  at  the 
lower  levels. 


130  OBSERVATIONAL  METEOROLOGY. 

CHAPTER    XXI. 

WINDS. 

THE  determination  of  the  direction  of  the  wind  (by  means  of 
the  wind  vane)  and  of  its  velocity  (by  means  of  the  anemome- 
ter, or  by  estimating  its  strength)  at  different  hours,  under  dif- 
ferent conditions  of  weather  and  in  different  seasons,  leads  to  a 
number  of  problems.  The  following  simple  investigations  may 
readily  be  undertaken  in  schools  :  — 

A.  The  Diurnal  Variation  in  Wind  Velocity  in  Fair  Weather.  — 
Observe  and  record  the  velocity  of  the  wind  (either  estimated 
or  registered    by  the  anemometer)  every  hour,  or  as  often  as 
possible,  on  clear  or  fair  days  in  different  months.     Can  you 
discover  any  regular  change  in  the  velocities  during  the  day  ? 
If  so,  what  is  the  change  ?     Does  the  season  seem  to  have  any 
control  over  the  results  obtained?     Examine  the  daily  weather 
maps  in  connection  with  your  observations  and  determine  the 
effect  that  different  weather  conditions  have  upon  the  diurnal 
variation  in  wind  velocity. 

The  diurnal  variation  in  wind  velocity  over  the  open  ocean  is  so 
slight  as  hardly  to  be  noticeable.  Over  the  land,  the  daytime  winds 
are  commonly  strongest  in  arid  regions.  Traveling  across  the 
desert  often  becomes  extremely  disagreeable,  owing  to  the  clouds  of 
dust  which  these  winds  sweep  up  from  the  surface. 

B.  The  Variations  in  Direction  and  Velocity  due  to  Cyclones  and 
Anticyclones.  —  Record  the  direction  and  velocity  of  the  wind  at 
your  station  at  frequent  intervals  during  the  passage  of  a  con- 
siderable number  of  cyclones  and  anticyclones.       Enter  your 
observations  in  some  form  of  table  so  that  they  may  be  readily 
examined.    (See  p.  113.)    Note  the  character  of  the  changes  that 
occur,  classifying  them  into  types,  so  far  as  possible.     Study  the 
control  of  wind  directions  and  velocities  by  the  special  features 
of  the  individual  cyclones  and  anticyclones  as  shown  on  the 


WINDS.  181 

daily  weather  maps.  How  are  the  different  types  of  change 
in  direction  and  velocity  affected  by  the  tracks  of  cyclones 
and  anticyclones  ?  By  their  velocity  of  progression  ?  By  the 
arrangement  of  isobars  around  them?  By  the  height  of  the 
barometer  at  the  center  ?  By  the  season  in  which  the  cyclones 
and  anticyclones  occur? 

Frequent  changes  in  the  direction  and  velocity  of  our  winds  are 
one  great  characteristic  of  the  Temperate  Zones,  especially  in  winter. 
The  continuous  procession  of  cyclones  and  anticyclones  across  the 
United  States  involves  continuous  shifts  of  wind.  Over  much  of 
the  earth's  surface,  however,  the  regularity  and  constancy  of  the 
winds  are  the  distinguishing  feature  of  the  climate.  Over  a  con- 
siderable part  of  the  belts  blown  over  by  the  northeast  and  southeast 
trades,  roughly  between  latitude  30°  N.  and  S.  and  the  equator,  the 
winds  keep  very  nearly  the  same  direction  and  the  same  velocity 
day  after  day  and  month  after  month.  Thus  the  trades  are  of  great 
benefit  to  commerce.  Sailing  ships  may  travel  for  days  in  the  trade 
wind  belts  without  having  their  sails  shifted  at  all,  with  a  fail- 
wind  all  the  time  carrying  them  rapidly  on  to  their  destination. 

C.  The  Occurrence  and  Characteristics  of  Local  Winds,  such  as 
Mountain  and  Valley  and  Land  and  Sea  Breezes.  —  If  the  observer 
happens  to  be  living  in  or  near  the  mouth  of  a  valley 
or  on  a  mountain  side,  opportunity  may  be  given  for  the 
observation  of  the  local  winds  down  the  mountain  sides  and 
down  the  valley  at  night,  and  up  the  valley  and  the  mountain 
sides  by  day,  known  as  mountain  and  valley  breezes.  Keep  a 
record  of  wind  direction  and  velocity  during  the  day,  and 
especially  during  the  morning  and  evening  hours.  Notice  any 
marked  changes  in  direction,  and  the  relation  of  these  changes 
to  the  time  of  day.  Does  the  velocity  of  the  daytime  up-cast 
breeze  show  any  systematic  variation  during  the  day  ?  Study 
the  relation  of  mountain  and  valley  breezes  to  the  general 
weather  conditions  shown  on  the  weather  maps.  How  are  these 
breezes  affected  by  season  ?  By  the  presence  of  a  cyclone  over 
the  region?  Of  an  anticyclone?  By  the  state  of  the  sky? 


132  OBSERVATIONAL  METEOROLOGY. 

If  near  the  seacoast  (i.e.,  within  10  or  15  miles),  an  interest- 
ing study  may  be  made  of  local  land  and  sea  breezes.  The  sea 
breeze  is  a  wind  from  the  ocean  onshore,  while  the  land  breeze 
blows  offshore.  These  breezes  occur  only  in  the  warmer 
months.  Take  frequent  observations  during  the  day,  as  in 
the  case  of  mountain  and  valley  winds,  noting  especially  any 
changes  in  direction  and  velocity,  and  the  relation  of  these 
changes  to  the  time  of  day.  Study  also  the  control  exercised 
by  the  prevailing  weather  conditions  over  the  occurrence  and 
the  strength  of  development  of  the  land  and  sea  breezes. 

This  problem  may  be  considerably  extended  by  adding  tem- 
perature observations  to  the  simpler  record  of  wind  direction 
and  velocity. 

In  some  of  the  Swiss  valleys  the  mountain  and  valley  breezes  are 
such  regular  daily  weather  phenomena  that  it  has  become  a  weather 
proverb  that  a  failure  of  the  daily  change  in  wind  direction  indi- 
cates a  change  of  weather.  Special  names  are  often  given  to  these 
breezes  where  they  are  well  marked.  In  a  part  of  the  Tyrol  sailing 
boats  go  up  the  lakes  by  day  with  the  valley  breeze,  and  sail  back 
at  night  with  the  mountain  breeze.  It  is  therefore  unnecessary  for 
the  boats  to  be  rowed  either  way.  Land  and  sea  breezes,  although 
an  unimportant  climatic  feature  in  these  northern  latitudes,  are 
often  of  the  highest  importance  in  the  Torrid  Zone.  The  fresh 
pure  sea  breeze  from  over  the  ocean  makes  it  possible  for  Europeans 
to  live  in  many  tropical  climates  where  otherwise  they  would  not 
keep  their  health.  The  land  breeze,  on  the  other  hand,  is  apt  to  be 
an  unhealthy  wind  in  the  tropics,  especially  when  it  blows  off  of 
swampy  land. 

CHAPTER   XXII. 

HUMIDITY,  DEW,  AND   FROST. 

THE  humidity  of  the  air,  as  determined  by  the  wet  and  dry 
bulb  thermometers  or  the  sling  psychrometer,  and  the  occur- 
rence or  absence  of  dew  or  frost,  should  be  studied  together. 


HUMIDITY,    DEW,    AND    FROST.  133 

Observations  should  be  made  at  different  hours,  in  different 
kinds  of  weather,  and  in  different  seasons.  From  such  observa- 
tions the  following  problems  may  be  solved  : 

A.  Diurnal   Variation  of  Relative   Humidity  under  Different 
Conditions,  —  Readings  of  the  wet  and  dry  bulb  thermometers 
in  the  instrument  shelter,  or  of  the  sling  psychrometer,  several 
times  during  the   day,  will  furnish  data  for  determining  the 
diurnal  variation  of  relative  humidity.     Classify  your  observa- 
tions according  to  the  weather  conditions  under  which  they 
were  made,  and  by  months  or  seasons.      Summarize  the  results 
of  your  investigation,  paying  special  attention  to  the  relation 
between   the   diurnal  variation  of   relative  humidity  and  the 
temperature. 

The  variations  of  relative  humidity  are  generally  the  reverse  of 
those  of  absolute  humidity.  In  the  case  of  the  latter  the  average 
diurnal  variations  are  small.  The  fluctuations  in  the  relative  humid- 
ity during  the  day  on  the  northwestern  coast  of  Europe  amount  to 
about  7%  in  December  and  17%  in  August,  while  in  central  Asia 
they  average  about  25%  in  winter  and  50%  in  summer. 

B.  Relation  of  Relative  Humidity  to  the  Direction  of  the  Wind. 
—  Observations  by  means  of  the  wet  and  dry  bulb  thermom- 
eters in  the  shelter,  or  by  means  of  the  sling  psychrometer, 
supplemented  by  records  of  wind  direction,  will  furnish  data 
for  the  solution  of  this  problem.      Tabulate  your  observations 
according   to   wind   directions    and   seasons.      Determine    the 
characteristics    of    the    different    winds    as   to   their    relative 
humidities.     Consider  the  control  of  these  winds  and  humidity 
conditions  by  cyclones  and  anticyclones. 

The  warm  wave,  or  sirocco,  in  front  of  our  winter  cyclones  in  the 
eastern  United  States  is  a  damp,  disagreeable,  irritating  wind.  In 
summer,  the  sirocco  is  usually  dry,  and  during  the  prevalence  of 
such  winds  we  have  our  hottest  spells,  when  sunstrokes  are  not  un- 
common. In  southern  Italy  the  sirocco  has  the  same  position  with 


134  OBSERVATIONAL    METEOROLOGY. 

reference  to  the  controlling  cyclone.  There  the  wind  is  often  so 
dry  as  seriously  to  injure  vegetation.  The  cold  wave,  on  the  rear 
of  our  winter  cyclones,  with  its  low  temperature  and  dry  air,  often 
comes  as  a  refreshing  change  after  the  enervating  warmth  of  the 
preceding  sirocco.  Our  feelings  of  bodily  comfort  or  discomfort 
are  thus  in  a  large  measure  dependent  upon  the  humidity  and  the 
movement  of  the  air. 

C.  The  Formation  of  Dew.  —  The  formation  of  dew  is  to  be 
studied  from  the  following  points  of  view,  viz.,  as  dependent 
upon  :  a,  the  temperature  and  the  humidity  of  the  air  ;  6,  the 
exposure  and  condition  of  the  ground ;  c,  the  state  of  the  sky  ; 
and  d,  the  movement  of  the  air.  The  occurrence  of  dew  on 
any  night,  as  well  as  the  amount,  whether  large  or  small,  can 
readily  be  ascertained  by  inspection.  Observe  the  conditions 
of  temperature,  humidity,  cloudiness,  and  wind  direction  and 
velocity,  as  in  previous  exercises.  Pay  special  attention  to  the 
state  of  the  sky,  the  wind  movement,  and  the  vertical  distri- 
bution of  temperature  near  the  ground.  Under  heading  b 
(exposure  and  condition  of  the  ground)  make  observations  of 
the  amounts  of  dew  formed  on  hilltops,  hillsides,  and  in  valleys  ; 
on  different  kinds  of  surface  covering,  as  grass,  leaves,  pave- 
ments, etc.,  and  over  different  kinds  of  soil.  Classify  the 
results  in  accordance  with  the  conditions  under  which  the 
observations  were  made.  Compare  the  results  and  draw  your 
conclusions  from  this  study.  Practise  making  predictions  of 
the  formation  of  dew  in  different  places  and  under  different 
weather  conditions. 

Over  the  greater  portion  of  the  earth's  surface  the  amount  of 
dew  which  is  deposited  is  very  small.  It  has  been  estimated  that 
in  Great  Britain  the  total  annual  amount  would  measure  only  an 
inch  and  a  half  in  depth ;  and  in  central  Europe  the  depth  is  given 
as  hardly  one  inch.  In  some  parts  of  the  Torrid  Zone,  on  the  other 
hand,  dew  is  deposited  in  much  larger  quantities.  According  to 
Humboldt,  the  traveler  through  some  of  the  South  American  forests 


HUMIDITY,    DEW,    AND    FROST.  135 

often  finds  what  seems  to  be  a  heavy  shower  falling  under  the  trees, 
while  the  sky  is  perfectly  clear  overhead.  In  this  case  dew  is 
formed  on  the  tops  of  the  tree  in  sufficiently  large  quantities  to 
give  a  shower  underneath.  It  is  reported  that  on  the  Guinea  coast 
of  Africa  the  dew  sometimes  runs  off  the  roofs  of  the  huts  like  rain. 
In  many  dry  regions  the  dew  is  an  important  agency  in  keeping 
the  plants  alive. 

D.  The  Formation  of  Frost.  —  The  formation  of  frost  is  to  be 
studied  in  the  same  way  as  that  suggested  in  the  case  of  dew, 
i.e.,  as  dependent  upon  :  a,  the  temperature  and  the  humidity  of 
the  air  ;  5,  the  exposure  and  condition  of  the  ground  ;  <?,  the 
state  of  the  sky  ;  and  d,  the  movement  of  the  air.  Frosts  are 
usually  classified  as  light  or  heavy.  The  words  killing  frost  are 
also  used.  Study  .the  weather  and  surface  conditions  which 
are  most  favorable  to  the  formation  of  frost.  Pay  special  atten- 
tion to  the  relation  of  frost  and  inversions  of  temperature  ; 
to  the  frequency  of  frost  on  open  or  sheltered  surfaces ; 
on  hills  or  in  valleys,  and  on  the  lower  and  upper  branches 
of  trees  and  shrubs.  Determine,  as  well  as  you  can,  the 
weather  conditions  which  precede  light  or  heavy  frosts,  and 
make  predictions  of  coming  frosts,  when  the  conditions  warrant 
them. 

Our  Weather  Bureau  gives  much  attention  to  the  prediction  of 
frosts  and  to  the  prompt  and  widespread  distribution  of  frost  warn- 
ings. Growing  crops  and  fruits  are  often  seriously  injured  by 
frosts,  and  farmers  are  naturally  anxious  to  have  as  early  warning 
as  possible  of  their  occurrence.  Various  methods  of  protecting 
crops  and  trees  against  frost  are  used.  The  method  most  commonly 
employed  consists  in  the  building  of  fires  of  brush  or  other  inflam- 
mable material  on  the  windward  side  of  the  field  or  the  orchard 
when  a  frost  is  expected.  The  smoke  from  the  fire  is  blown  to  lee- 
ward across  the  field,  and  acts  as  an  artificial  cloud,  affording  pro- 
tection to  the  vegetation  underneath.  Such  fires  are  known  as 
smudges. 


136  OBSERVATIONAL  METEOROLOGY. 

CHAPTER    XXIII, 

CLOUDS   AND   UPPER   AIR   CURRENTS. 

ATTENTIVE  observation  of  clouds  will  soon  lead  to  a  familiar- 
ity with  their  common  type  forms.  A  series  of  cloud  views,1 
with  accompanying  descriptive  accounts,  will  teach  the  names 
of  the  clouds  and  give  defmiteness  to  the  record.  The  direc- 
tions of  movement  of  clouds  are  determined  by  means  of  the 
nephoscope.  Cloud  observations  should  be  made  at  different 
hours,  in  different  weather  conditions,  and  in  different  seasons. 
The  following  problems  are  concerned  with  clouds  and  upper 
air  currents  :  — 

A.  The  Typical  Cloud  Forms  and  their  Changes,  —  Note  care- 
fully the  characteristic  forms  assumed  by  clouds  ;  their  mode 
of  occurrence,  whether  in  single  clots,  or  in  groups,  in  lines, 
or  all  over  the  sky  ;   their  changes  in  form  and  in  mode  of 
occurrence.     Classify  and  summarize  your  results.     Compare 
the  clouds  of  the  warm  months  with  those  of  the  cold  months. 

Observations  have  shown  that  clouds  have  certain  definite  char- 
acteristic forms  which  are  substantially  the  same  in  all  parts  of  the 
world.  This  fact  makes  it  possible  to  give  names  to  the  different 
typical  forms,  and  these  names  are  used  by  observers  the  world 
over.  Hence  cloud  observations,  wherever  made,  are  comparable. 
The  first  classification  of  clouds  was  proposed  by  Luke  Howard, 
in  1803.,  The  classification  at  present  in  use  is  known  as  the 
International  Classification,  and  was  adopted  by  the  International 
Meteorological  Congress  in  1896. 

B.  The  Prevailing  Direction  of  Cloud  Movements.  —  The  use 

of  the  nephoscope  is  necessary  in  the  accurate  determination  of 
cloud  movements.  Study  the  prevailing  directions  of  move- 
ment of  the  clouds,  by  means  of  frequent  observations  with 
the  nephoscope,  in  different  weather  conditions.  Separate  the 

1  See  Hydrographic  Office  Cloud  Types,  Appendix  B. 


CLOUDS  AND  UPPER  AIR  CURRENTS.         137 

upper  and  lower  clouds  in  this  study.  Summarize  your 
results  according  to  the  weather  conditions  and  the  kinds  of 
clouds. 

C.  Correlation  of  Cloud  Form  and  Movement  with  Surface  Winds, 

with  Cyclones  and  Anticyclones,  and  with  Weather  Changes. The 

results  obtained  in  the  working  out  of  the  two  preceding  prob- 
lems   may  be  used  in    the   present  problem.     Tabulate  your 
observations  of  cloud  forms  with  reference  to  the  wind  direc- 
tions which  prevailed  at  the  time  of  making  the  observations. 
Do  the  same  with  the  directions  of  cloud  movement.     Deter- 
mine the    relation   between  surface    winds    and  cloud    types, 
and   between  surface   winds  and  the  direction  of  the   upper 
air    currents,    as    shown    by   the    movements    of    the    upper 
clouds.      Study  the   control  exercised   by  cyclones  and  anti- 
cyclones over  cloud  forms  and  over  the  direction  of  the  upper 
air  currents. 

D.  The   Use   of  Clouds  as  Weather  Prognostics Attentive 

observation  of  the  forms  and  changes  of  clouds,  and  of  the 
accompanying  and  following  weather  changes,  will  lead  to  the 
association  of  certain  clouds  with  certain  coming  weather  con- 
ditions.    Make  your  cloud  observations  carefully,  taking  full 
notes  at  the  time  of  observation.     Give  special  attention  to 
the  weather  conditions  that  follow.     Continue  this  investiga- 
tion through  as  long  a  period  as  possible,  until  you  have  gath- 
ered a  considerable  body  of  fact  to  serve  as  a  basis,  arid  then 
frame    a   set   of  simple   rules    for   forecasting   fair  or  stormy 
weather  on  the  basis  of  the  forms  and  changes  of  the  clouds. 
Such  local  observations  as  these  may  be  employed  as  a  help  in 
making  forecasts  from  the  daily  weather  maps. 

Clouds  were  used  as  weather  prognostics  long  before  meteoro- 
logical observations  and  weather  maps  were  thought  of.  To-day 
sailors  and  farmers  still  look  to  the  clouds  to  give  them  warning  of 
approaching  storms.  Many  of  our  common  weather  proverbs  are 
based  on  the  use  of  clouds  as  weather  prognostics. 


138  OBSERVATIONAL  METEOROLOGY. 

CHAPTER    XXIV. 

PRECIPITATION. 

THE  special  study  of  various  problems  connected  with  pre- 
cipitation involves  detailed  observations  of  the  amount  and 
rate  of  precipitation  of  various  kinds,  measured  by  the  rain 
gauge  during  storms  in  different  seasons.  These  observations 
of  precipitation  should,  of  course,  be  supplemented  by  the  usual 
record  of  the  other  weather  elements.  The  following  problems 
are  suggested  :  — 

A.  The  relation  of  precipitation  in  general  to  the  other  weather 
elements,  and  to  cyclones  and  anticyclones. 

B.  The  conditions  under  which  special  forms  of  precipitation 
(rain,  snow,  sleet,  hail,  frozen  rain)  occur. 

C.  The  conditions  associated  with  light  and  heavy,  brief  and 
prolonged,  local  and  general  rainfall. 

These  problems  are  studied  by  means  of  a  careful  comparison 
of  full  weather  records  with  the  daily  weather  maps  during 
a  considerable  period  of  time. 

Rain  is  the  most  common  form  of  precipitation  the  world  over, 
although  snow  falls  over  large  portions  of  both  hemispheres.  In 
the  Arctic  and  Antarctic  zones  almost  all  the  precipitation,  which  is 
small  in  amount,  comes  in  the  form  of  snow.  In  southern  Europe 
snow  falls  at  sea  level  during  the  winter  as  far  south  as  36°  north 
latitude  on  the  average.  In  eastern  Asia  snow  occasionally  falls  as  far 
south  as  23°  north  latitude.  The  mean  annual  rainfall  varies  greatly 
in  different  parts  of  the  world.  In  desert  regions  it  is  practically 
nothing.  At  Cherrapunjee,  in  India,  it  reaches  493  inches,  or  over 
40  feet.  A  fall  of  40.8  inches  in  a  single  day  occurred  at  this 
station  on  June  14,  1876.  In  the  United  States,  Upper  Mattole, 
Cal.,  had  an  extraordinary  monthly  rainfall  of  41.63  inches  in  Jan- 
uary, 1888.  An  excessive  daily  rainfall  of  8  inches  occurred  at 
Syracuse,  1ST.  Y.,  on  June  8,  1876.  At  Washington,  D.  C.,  2.34 


PRESSURE.  139 

inches  fell  in  37  minutes  on  June  27,  1881.  A  sudden  and  very 
heavy  fall  of  rain  occurred  at  Palmetto,  Nevada,  in  August,  1890. 
A  rain  gauge  which  was  not  exposed  to  the  full  intensity  of  the 
storm  caught  8.80  inches  of  water  in  one  hour.  In  August,  1891, 
an  observer  at  Campo,  Cal.,  measured  11.5  inches  as  the  rainfall 
in  one  hour  from  one  very  heavy  downpour,  and  from  a  portion 
of  a  second  storm. 


CHAPTER   XXV. 

PRESSURE. 

THE  variations  of  atmospheric  pressure,  although  insensible 
to  non-instrumental  observation,  are  so  intimately  connected 
with  atmospheric  processes  that  they  deserve  careful  attention. 
Their  observation  leads  to  several  problems. 

A.  The  Decrease  of  Pressure  with  Height,  as  between  Valley 
and  Hill,  or  between  the  Base  and  Top  of  a  Building.  —  Make 
these  observations  with  the  mercurial  barometer,  if  possible. 
Note  the  air  temperatures  at  the  two  levels  at  which  the  barom- 
eter readings  are  made.  Determine  the  heights  of  hill  or 
building  by  means  of  the  following  rule :  Multiply  by  9  the 
difference  in  barometrical  readings  at  the  two  stations,  given  in 
hundredths  of  an  inch,  and  the  result  will  be  approximately  the 
difference  in  height  between  the  stations  in  feet.  A  more 
accurate  result  may  be  reached  by  means  of  the  following  rule : 
The  difference  of  level  in  feet  is  equal  to  the  difference  of  the 
pressures  in  inches  divided  by  their  sum,  and  multiplied  by  the 
number  55,761,  when  the  mean  of  the  air  temperatures  of 
the  two  places  is  60°.  If  the  mean  temperature  is  above  60°, 
the  multiplier  must  be  increased  by  117  for  every  degree  by 
which  the  mean  exceeds  60° ;  if  less  than  60°,  the  multiplier 
must  be  decreased  in  the  same  way.  For  example,  if  the  lower 


140  OBSERVATIONAL    METEOROLOGY. 

station  has  a  pressure  of  30.00  inches  and  a  temperature  of  62°, 
and  the  upper  station  has  29.00  inches  and  58°  respectively,  the 
difference  of  level  between  the  two  will  be 

30.00  -  29.00 

30.00  +  29.00  X  55'761 

If  the  lower  values  are  30.15  inches  and  65°,  while  the  upper 
values  are  28.67  inches  and  59°,  then  the  formula  becomes 


(2  x  m)]  =  u°9  feet- 


The  determination  of  heights  by  means  of  the  barometer  depends 
upon  the  fact  that  the  rate  of  decrease  of  pressure  upwards  is 
known.  As  the  weight  of  a  column  of  air  of  a  given  height  varies 
with  the  temperature  of  the  air,  it  is  necessary,  in  accurate  work  of 
this  sort,  to  know  the  air  temperatures  at  both  the  lower  and  upper 
stations  at  the  time  of  observation.  From  these  temperatures  the 
mean  temperature  of  the  air  column  between  the  two  stations 
may  be  determined.  Tables  have  been  published  which  facilitate 
the  reductions  in  this  work.  The  heights  of  mountains  are  usu- 
ally determined,  in  the  first  instance,  by  means  of  barometric  ob- 
servations, carried  out  by  scientific  expeditions  or  by  travelers  that 
have  been  able  to  reach  their  summits.  More  accurate  measure- 
ments are  later  made,  when  possible,  by  means  of  trigonometrical 
methods. 

B.  The  Diurnal  and  Cyclonic  Variation  of  Pressure  in  Different 
Seasons.  —  This  problem  is  satisfactorily  solved  only  by  a  study 
of  the  curves  traced  by  the  barograph,  or  by  plotting,  as  a 
curve,  hourly  or  half-hourly  readings  of  the  mercurial  barometer. 
The  diurnal  variation  of  the  barometer  is  the  name  given  to  a 
slight  double  oscillation  of  pressure,  with  two  maxima  and 
two  minima  occurring  during  the  24  hours.  This  oscilla- 
tion is  in  some  way,  not  yet  understood,  connected  with  the 
diurnal  variation  in  temperature.  It  is  most  marked  in  the 
tropics  and  diminishes  towards  the  poles.  Fig.  15  illustrates, 
in  the  May  curve,  the  diurnal  variation  of  the  barometer  at 


PRESSURE.  141 

Cambridge,  Mass.,  during  a  spell  of  fair  spring  weather,  May 
18-22,  1887.  The  maxima  are  marked  by  +  and  the  minima 
by  0.  The  cyclonic  variation  of  pressure  is  the  name  given  to 
those  irregular  changes  in  pressure  which  are  caused  by  the 
passage  of  cyclones  and  anticyclones.  The  second  curve  in 
Fig.  15  shows  the  cyclonic  variations  in  pressure  recorded  by 
the  barograph  at  Cambridge,  Mass.,  during  a  spell  of  stormy 
weather,  Feb.  23-28,  1887.  These  curves  serve  as  good  illus- 
trations of  these  two  kinds  of  pressure  variations. 

Study  your  barograph  tracings,  or  your  barometer  readings,  as 
illustrating  diurnal  or  cyclonic  variations  of  pressure.  Note 
the  character  and  the  amount  of  the  diurnal  and  cyclonic  varia- 
tions, and  their  dependence  on  seasons. 

Over  the  greater  part  of  the  Torrid  Zone  the  diurnal  variation  of 
the  barometer  is  remarkably  distinct  and  regular.  Hum  bold  t  first 
called  attention  to  the  fact  that  in  those  latitudes  the  tune  of  day 
may  be  told  within  about  15  minutes  if  the  height  of  the  barometer 
is  known. 

0.  The  Relation  of  Local  Pressure  Changes  to  Cyclones  and 
Anticyclones,  and  thus  to  Weather  Changes.  —  Make  a  detailed 
study  of  the  relation  of  the  local  pressure  changes  at  your 
station,  as  shown  by  the  barograph  curves,  or  by  frequent  read- 
ings of  the  mercurial  or  aneroid  barometers,  to  the  passage  of 
cyclones  an4  anticyclones,  and  to  their  accompanying  weather 
changes.  Classify  the  simple  types  of  pressure  change,  so  far 
as  possible,  together  with  the  general  weather  conditions  that 
usually  accompany  these  types.  Apply  the  knowledge  of  local 
weather  changes  thus  gained  when  you  make  your  forecast  on 
the  basis  of  the  daily  weather  maps. 


142  OBSEBVATIONAL  METEOROLOGY. 

CHAPTER  XXVI. 

METEOROLOGICAL   TABLES. 

THE  tables  which  follow  are  those  which  are  now  in  use  by 
the  United  States  Weather  Bureau.  They  were  first  published 
in  the  Instructions  for  Voluntary  Observers  issued  in  1892, 
and  were  reprinted  in  1897.  The  following  instructions  will 
be  found  of  service  in  the  use  of  the  tables  :  — 

% 

TABLE    I.  —  DEW-POINT. 

The  figures  in  heavy  type,  arranged  in  vertical  columns  at 
each  side  of  the  page,  are  the  air  temperatures  in  degrees  Fah- 
renheit, as  recorded  by  the  dry-bulb  thermometer.  The  figures 
in  heavy  type,  running  across  the  page,  denote  the  differences, 
in  degrees  and  tenths  of  degrees,  between  the  dry-bulb  and 
wet-bulb  readings,  or,  technically,  the  depression  of  the  wet-bulb 
thermometer.  The  figures  in  the  vertical  columns  denote  the 
dew-points.  Make  your  observation  of  the  wet  and  dry  bulb 
thermometers  and  note  the  difference  between  the  two  readings. 
Find,  in  the  vertical  columns  of  heavy  type,  the  temperature 
corresponding  to  your  dry-bulb  reading,  or  the  nearest  tempera- 
ture to  that.  Then  look  along  the  horizontal  lines  of  figures  in 
heavy  type  for  the  figure  which  corresponds  exactly,  or  most 
nearly,  with  the  difference  between  your  wet  and  dry  bulb 
readings.  Look  down  the  vertical  column  under  this  latter 
figure  until  you  reach  the  horizontal  line  corresponding  to  your 
dry-bulb  reading.  At  this  point  the  figures  in  the  vertical 
column  give  the  dew-point  of  the  air  at  the  time  of  your 
observation. 

Example  :  Air  Temperature  (dry  bulb),  47°;  Wet  Bulb,  44° ; 
Difference,  8°.  On  page  148  will  be  found  the  table  containing 


METEOROLOGICAL   TABLES.  143 

both  47°  (dry  bulb)  and  3°  (depression  of  the  dew-point).  In 
the  twenty-eighth  line  of  this  table  and  in  the  seventh  column 
will  be  found  the  dew-point,  viz.,  41°. 

Example:  Air  Temperature,  61.5°;  Wet  Bulb,  55.5°;  Differ- 
ence, 6°. 

In  this  case  61.5°  is  not  found  in  the  vertical  columns  of 
dry-bulb  readings,  but  61°  and  62°  are  found.  The  dew-point, 
with  a  difference  between  wet  and  dry  bulb  readings  of  6°,  for 
an  air  temperature  of  61°,  is  50°  ;  for  an  air  temperature  of  62°, 
it  is  52°.  Evidently,  then,  for  an  air  temperature  of  61.5°  the 
dew-point  will  be  51°,  i.e.,  halfway  between  50°  and  52°. 
This  method  of  determining  dew-points  at  air  temperatures  or 
with  depressions  of  the  wet-bulb  thermometer  which  are  not 
given  exactly  in  the  tables,  is  known  as  interpolation. 

Example:  Air  Temperature,  93°;  Wet  Bulb,  90.5°  ;  Differ- 
ence, 2.5°.  Our  table  gives  no  dew-points  for  wet-bulb  depres- 
sions of  2.5°,  with  air  temperature  93°,  but  we  find  (on  page 
152)  that  for  air  temperature  93°  and  depression  of  wet  bulb 
of  2°,  the  dew-point  is  91°,  while  for  a  wet-bulb  depression  of 
3°,  the  dew-point  is  89°.  By  the  method  of  interpolation  we 
can  readily  determine  the  dew-point  in  the  special  case  under 
consideration  as  90°,  i.e.,  halfway  between  89°  and  91°. 

TABLE  II.  —  RELATIVE  HUMIDITY. 

The  general  plan  of  this  table  is  the  same  as  that  of  Table  I. 
The  figures  in  the  vertical  columns  are  the  relative  humidities 
(in  percentages)  corresponding  to  the  different  readings  of  the 
wet  and  dry  bulb  thermometers. 

TABLE  III.  — REDUCTION  OF  BAROMETER  TO  32°. 

The  figures  in  heavy  type,  arranged  in  vertical  columns  at 
the  left  of  the  page,  refer  to  the  temperature  in  degrees  Fah- 
renheit, as  indicated  by  the  attached  thermometer.  The  figures 


144  OBSERVATIONAL  METEOROLOGY. 

in  heavy  type,  running  across  the  top  of  the  page,  are  the 
barometer  readings  in  inches  and  tenths.  Make  a  reading 
of  the  attached  thermometer  and  of  the  barometer.  Find  in 
the  vertical  column  the  temperature  corresponding  to  the  read- 
ing of  the  attached  thermometer,  and  in  the  horizontal  line  of 
heavy  figures  the  reading  corresponding  to  the  height  of  the 
barometer.  The  decimal  in  the  vertical  column,  under  the 
appropriate  barometer  reading,  and  in  the  same  horizontal  line 
with  the  appropriate  thermometer  reading,  is  to  be  subtracted 
from  the  height  of  the  barometer  as  observed,  thus  correcting 
the  reading  to  freezing.  When  the  attached  thermometer  reads 
below  28°,  the  correction  is  additive. 

Example :  Attached  Thermometer,  69°  ;  Barometer,  30.00 
inches  ;  Correction,  —  .110  ;  Corrected  reading,  29.890  inches. 

Example:  Attached  Thermometer,  73°;  Barometer,  29.75 
inches  ;  Correction  =  ? 

We  do  not  find  any  column  corresponding  to  a  barometer 
reading  of  29.75  inches.  We  do  find,  however,  that  with  a 
barometer  reading  of  29.50,  and  an  attached  thermometer  read- 
ing of  73,  the  correction  is  —  .118  inch,  and  with  a  barometer 
reading  of  30.00,  the  correction  is  —.120.  By  interpolating,  as 
in  the  case  of  the  humidity  table  above,  we  find  the  correction 
for  a  barometer  reading  of  29.75  inches,  and  an  attached  ther- 
mometer reading  of  73°.  The  correction  is  —  .119,  and  the 
corrected  reading  is  29.75  —  .119  =  29.63  inches. 

TABLE  IV.  —  REDUCTION  OF  BAROMETER  TO  SEA  LEVEL. 

The  figures  in  heavy  type,  in  the  left-hand  vertical  columns, 
are  the  heights,  in  feet,  of  the  barometer  above  sea  level.  The 
figures  in  heavy  type  at  the  top  of  the  columns,  running  across 
the  page,  are  the  readings  of  the  ordinary  thermometer.  The 
numbers  of  inches  and  hundredths  of  inches  to  be  subtracted 
from  the  barometer  reading  (corrected  for  temperature  by  Table 


METEOROLOGICAL   TABLES.  145 

III),  for  the  different  heights  above  sea  level,  are  given  in  the 
vertical  columns. 

The  altitude  above  sea  level  of  the  city  or  town  at  which 
the  observation  is  made  should  be  ascertained  as  accurately  as 
possible  from  some  recognized  authority,  as,  e.g.,  from  a  rail- 
road survey ;  from  Government  measurements,  or  from  some 
engineer's  office.  The  correction  to  be  made  is  determined 
by  a  simple  inspection  of  the  table  or  by  the  method  of  inter- 
polation. 

Example :  Altitude  of  Barometer  above  sea  level,  840  feet ; 
Temperature  of  the  air,  40°  ;  Correction,  +.931  inch. 

Example :  Altitude  of  Barometer  above  sea  level,  205  feet ; 
Temperature  of  the  air,  45°  ;  Correction  =  ? 

Here  205  feet  and  45°  are  neither  of  them  found  in  the 
table.  Hence  a  double  interpolation  is  necessary.  For  200 
feet  and  40°  the  correction  is  +  .224  inch.  For  200  feet 
and  50°  the  correction  is  +  .220  inch.  Hence  for  200  feet  and 
45°  the  correction  is  +.222  inch.  For  210  feet  and  40° 
the  correction  is  +  .235  inch.  For  210  feet  and  50°  the 
correction  is  +.231  inch.  Hence  for  210  feet  and  45°  the 
correction  is  +  .233  inch.  Now  for  205  feet  we  should  have 
a  correction  midway  between  +  .235  inch  and  +  .233  inch  or 
+  .234  inch. 


TABLE  I. — TEMPERATURE  OF  THE  DEW-POINT,  IN  DEGREES  FAHRENHEIT. 


0> 

a 

Difference  between  the  dry  and  wet  thermometers  (t—f). 

h 

2 

1 

Oo.2 

0°.4 

00.6 

00.8 

1°.0 

1°.2 

lo.4 

lo.6 

1°.8 

20.0 

20.2 

20.4 

2o.6 

£ 

-40 

-52 

-40 

-39 

-50 

-39 

-38 

-49 

-38 

-37 

-48 

-37 

-36 

-46 

—36 

-35 

44 

-35 

-34 

-43 

-58 

-34 

-33 

-42 

-55 

-33 

-32 

-40 

-52 

-32 

-31 

-38 

-49 

-31 

-30 

-36 

-47 

-30 

-29 

-35 

-44 

-29 

—  28 

-33 

-42 

-56 

-28 

-27 

-32 

—40 

-52 

-27 

-26 

-30 

-37 

-48 

-26 

—  25 

-29 

-35 

-45 

-25 

-24 

-28 

-34 

-43 

-58 

-24 

-23 

-27 

-32 

-40 

-53 

-23 

—  22 

-26 

-30 

-37 

-49 

-22 

-21 

-25 

-29 

-35 

-45 

-21 

-20 

-23 

-28 

-33 

—41 

-55 

-20 

-19 

-22 

-26 

-31 

-38 

-50 

-19 

-18 

-2t 

-25 

-29 

-35 

-45 

-18 

—  17 

-20 

-23 

-27 

-32 

—  41 

-55 

-17 

-16 

-19 

-22 

-26 

-30 

-37 

-49 

-16 

-15 

-17 

-20 

-24 

-28 

-34 

-44 

-15 

-14 

-16 

-19 

-22 

-26 

—31 

-39 

-52 

-14 

-13 

-15 

-18 

-21 

-25 

-29 

-35 

-46 

-13 

-12 

-14 

-17 

-20 

-23 

-27 

-32 

-41 

-55 

-12 

-11 

-13 

-16 

-18 

-21 

-25 

-30 

-36 

-48 

-11 

-10 

-12 

-14 

-17 

-20 

-23 

-27 

-33 

-42 

-58 

—  10 

-  9 

-11 

-13 

-15 

-18 

-21 

-25 

-30 

-37 

-48 

-  9 

-   8 

-10 

-12 

-14 

-17 

—20 

-23 

-27 

-33 

-42 

-58 

-   8 

-   7 

—  9 

-11 

-13 

-15 

-18 

-21 

-25 

-30 

-36 

-48 

-   7 

g 

-  8 

-10 

-12 

-14 

-16 

-19 

-23 

—27 

-32 

-41 

—56 

-   6 

-  5 

-  7 

-  8 

-10 

-12 

-15 

—17 

-21 

-24 

—29 

-35 

-47 

-   5 

-   4 

-   6 

-  7 

Q 

-11 

-13 

-16 

-19 

-22 

-26 

-31 

-39 

-54 

-  4 

-   3 

-  4 

—  6 

—  8 

-10 

-12 

-14 

-17 

-20 

-23 

-28 

-33 

-44 

-   3 

-  2 

-  3 

—  5 

—  6 

-  8 

-10 

-12 

-15 

-18 

-21 

-24 

-29 

-36 

-48 

—   2 

—  1 

-  2 

-  4 

-  5 

-  7 

-  9 

-11 

—  13 

-16 

-18 

-22 

-26 

-31 

-39 

-   1 

±  0 

-  1 

-  3 

—  4 

—  6 

—  7 

-  9 

—  11 

-14 

-16 

-19 

-23 

-27 

-33 

0 

-I-  1 

-  0 

—  2 

-  3 

-  4 

-  6 

-  8 

-10 

-12 

—  14 

-17 

-20 

-24 

-28 

+  1  • 

2 

+  1 

-  1 

-  2 

-  3 

—  5 

-  6 

-  8 

-10 

-12 

-15 

-17 

-21 

-25 

2 

3 

2 

+  1 

-  1 

-  2 

-  3 

-  5 

—  7 

-  8 

-10 

-13 

-15 

-18 

-21 

3 

4 

3 

+  2 

0 

-  1 

-  2 

-  4 

-  5 

-  7 

-  9 

-11 

-13 

-16 

-19 

4 

5 

4 

3 

+  1 

0 

-  1 

2 

-  4 

-  5 

7 

-  9 

-11 

-14 

-16 

5 

6 

5 

4 

3 

+  1 

0 

—  1 

-  3 

—  4 

—  6 

-  7 

q 

-12 

-14 

6 

7 

6 

5 

4 

3 

+  1 

0 

1 

-  3 

—  4 

-  6 

-  8 

-10 

-12 

7 

8 

7 

6 

5 

4 

3 

+  1 

0 

-   1 

—  3 

-  4 

—  6 

-  8 

-10 

8 

9 

8 

7 

6 

5 

4 

3 

+  1 

0 

—  1 

-  3 

—  4 

-  6 

-  8 

9 

10 

9 

8 

7 

6 

5 

4 

3 

+  1 

0 

_  j 

-  3 

-  4 

-  6 

10 

11 

10 

9 

8 

7 

6 

5 

4 

3 

+  2 

0 

—  1 

0 

-  4 

11 

12 

11 

10 

9 

9 

8 

7 

5 

4 

3 

+  2 

0 

—  1 

—  2 

12 

13 

12 

11 

11 

10 

9 

8 

7 

6 

5 

3 

+  2 

+  1 

—  1 

13 

14 

13 

12 

12 

11 

10 

9 

8 

7 

6 

5 

4 

2 

+  1 

14 

15 

14 

13 

13 

12 

11 

10 

9 

8 

7 

6 

5 

4 

3 

15 

16 

15 

15 

14 

13 

12 

11 

10 

10 

9 

8 

7 

5 

4 

16 

17 

16 

16 

15 

14 

13 

12 

12 

11 

10 

9 

8 

7 

6 

17 

18 

17 

17 

16 

15 

14 

14 

13 

12 

11 

10 

9 

8 

7 

18 

19 

18 

18 

17 

16 

15 

15 

14 

13 

12 

11 

10 

10 

9 

19 

20 

19 

19 

18 

17 

17 

16 

15 

14 

13 

13 

12 

11 

10 

20 

t. 

00.2 

0°.4 

Oo.6 

00.8 

lo.O 

10.2 

1°.4 

10.6 

lo.8 

2o.O 

20.2 

20.4 

2o.6 

t. 

146 


METEOROLOGICAL   TABLES. 


147 


TABLE  I.  —  TEMPEKATURE  OF  THE  DEW-POINT,  IN  DEGREES  FAHRENHEIT. 


i 

Difference  between  the  dry  and  wet  thermometers  (t—f). 

I 

"t 

2o.« 

20.8 

3o.O 

3o.2 

3o.4 

3°.6 

30.8 

4°.0 

40.2 

40.4 

"t 

—  2 

-48 

2 

-   1 

-39 

-54 

-  1 

0 

—33 

-43 

0 

+  1 

-28 

-35 

-46 

a 

-25 

-30 

-37 

-50 

2 

3 

-21 

-26 

-31 

-39 

-54 

3 

4 

-19 

-22 

-27 

-32 

-42 

-60 

4 

5 

-16 

-19 

-23 

—  28 

-34 

-45 

5 

6 

-14 

-17 

-20 

-24 

-29 

-35 

-47 

6 

7 

-12 

-14 

-17 

-20 

-24 

-29 

-37 

-50 

7 

8 

-10 

-12 

-15 

-17 

-21 

-25 

-30 

-38 

-53 

8 

9 

-  8 

-10 

-12 

-15 

-18 

-21 

-25 

-31 

-39 

-55 

9 

10 

-  6 

-  8 

-10 

-12 

-15 

-18 

-21 

-26 

-31 

-40 

10 

11 

-  4 

-  6 

-  8 

-10 

-12 

-15 

-18 

-21 

-26 

-31 

11 

12 

-  2 

-  4 

-  6 

-  8 

-10 

12 

-15 

-18 

-21 

—26 

12 

13 

—  1 

-  2 

-  4 

-  5 

-  7 

-  9 

-12 

-14 

-17 

-21 

13 

14 

+  1 

0 

-  2 

-  3 

-  5 

-  7 

-  9 

-11 

-14 

-17 

14 

15 

3 

+  1 

0 

-  2 

-  3 

-  5 

-  7 

-  9 

-11 

-14 

15 

16 

4 

3 

+  2 

0 

-  1 

-  3 

-  4 

-  6 

-  8 

-10 

16 

17 

6 

5 

3 

+  2 

+  1 

t 

-  2 

-  4 

-  6 

-  8 

17 

18 

7 

6 

5 

4 

2 

+  1 

0 

-  2 

-  3 

-  5 

18 

19 

9 

8 

7 

5 

4 

3 

+  1 

0 

-  3 

19 

20 

10 

9 

8 

7 

6 

5 

3 

+  2 

+  1 

—  1 

20 

t. 

2°.  6 

20.8 

3o.« 

30.2 

™ 

30.6 

30.8 

40.0 

40.2 

40.4 

t. 

t. 

40.6 

40.8 

6°.0 

50.2 

5°.4 

50.6 

5°.8 

6o.O 

6o.2 

6°.4 

t. 

8 

8 

9 

9 

10 

-57 

10 

11 

-41 

-60 

11 

12 

-31 

-41 

-59 

12 

13 

-25 

-31 

-40 

-58 

13 

14 

-20 

-25 

-30 

-39 

-56 

14 

15 

-16 

-20 

-24 

-30 

-38 

-53 

15 

16 

*-13 

—16 

-19 

-23 

-28 

-36 

-50 

16 

17 

-10 

-12 

—15 

-18 

-22 

-27 

-34 

-47 

17 

18 

_  7 

_  9 

—  12 

—14 

-17 

-21 

-26 

-32 

—44 

18 

19 

-  5 

-  7 

-  9 

-11 

-14 

-17 

-20 

-25 

-30 

-40 

19 

20 

—  2 

-  4 

-  6 

—  8 

—10 

-13 

-16 

-19 

-23 

-29 

20 

1. 

4°.6 

4°.8 

5o.O 

50.2 

60.4 

6°.6 

50.8 

6°.0 

60.2 

6o.4 

t. 

TABLE  I.  —  TEMPERATURE  OF  THE  DEW-POINT,  IN  DEGREES  FAHRENHEIT. 


«' 

Difference  between  the  dry  and  wet  thermometers  (t  —  t'). 

S 

-5 

1 

0°.5 

lo.O 

1°.5 

2°.0 

20.5 

3°.0 

t 

30.5 

40.0 

40.5 

50.0 

60.5 

6o.O 

| 

20 

18  ' 

17 

15 

13 

10 

8 

5 

2 

-2 

6 

—12 

-19 

20 

21 

19 

18 

16 

14 

12 

9 

7  " 

4 

0 

-4 

—  8 

-15 

21 

22 

20 

19 

17 

15 

13 

11 

8 

6 

+  2 

_  i 

—  6 

-11 

22 

23 

22 

20 

18 

16 

14 

12 

10 

7 

4 

+  1 

-  3 

-  8 

23 

24 

23 

21 

19 

18 

16 

14 

11 

9 

6 

3 

—  1 

-  5 

24 

25 

24 

22 

21 

19 

17 

15 

13 

11 

8 

5 

+  2 

-  2 

25 

26 

25 

23 

22 

20 

18 

16 

14 

12 

10 

7 

4 

0 

26 

27 

26 

24 

23 

21 

20 

18 

16 

14 

11 

9 

6 

+  3 

27 

28 

27 

25 

24 

22 

21 

19 

17 

15 

13 

11 

8 

5 

28 

29 

28 

26 

25 

24 

22 

20 

19 

17 

14 

12 

10 

7 

29 

30 

29 

27 

26 

25 

23 

22 

20 

18 

16 

14 

11 

9 

30 

31 

30 

29 

27 

26 

24 

23 

21 

19 

18 

15 

13 

11 

31 

32 

31 

30 

28 

27 

26 

24 

22 

21 

19 

17 

15 

13 

32 

33 

31 

31 

29 

28 

26 

25 

23 

22 

19 

18 

16 

14 

33 

34 

32 

32 

30 

29 

27 

26 

24 

24 

21 

20 

18 

16 

34 

35 

33 

32 

31 

30 

29 

28 

26 

25 

23 

22 

20 

18 

35 

36 

35 

34 

32 

31 

30 

29 

27 

26 

24 

23 

21 

19 

36 

37 

36 

35 

33 

32 

31 

30 

28 

27 

26 

24 

22 

21 

37 

38 

37 

36 

34 

33 

32 

31 

30 

28 

27 

26 

24 

22 

38 

39 

38 

37 

35 

34 

33 

32 

30 

29 

28 

27 

25 

24 

39 

40 

39 

38 

36 

35 

34 

33 

31 

30 

29 

28 

26 

25 

40 

41 

40 

39 

37 

36 

35 

34 

32 

32 

30 

29 

28 

26 

41 

42 

41 

40 

39 

38 

36 

35 

34 

33 

31 

30 

29 

27 

42 

43 

42 

41 

40 

39 

37 

36 

35 

34 

32 

31 

30 

29 

43 

44 

43 

42 

41 

40 

38 

37 

36 

35 

33 

32 

31 

30 

44 

45 

44 

43 

42 

41 

40 

39 

37 

36 

34 

33 

32 

31 

45 

46 

45 

44 

43 

42 

41 

40 

38 

37 

36 

35 

33 

32 

46 

47 

46 

45 

44 

43 

42 

41 

40 

39 

37 

36 

34 

33 

47 

48 

47 

46 

45 

44 

43 

42 

41 

40 

38 

37 

36 

35 

48 

49 

48 

47 

46 

45 

44 

43 

42 

41 

39 

38 

37 

36 

49 

50 

49 

48 

47 

46 

45 

44 

43 

42 

41 

40 

38 

37 

50 

51 

50 

49 

48 

47 

46 

45 

44 

43 

42 

41 

39 

38 

51 

52 

51 

50 

49 

48 

47 

46 

45 

44 

43 

42 

41 

40 

52 

53 

52 

51 

50 

49 

48 

47 

46 

45 

44 

43 

42 

41 

53 

54 

53 

52 

51 

50 

50 

49 

47 

46 

45 

44 

43 

42 

54 

55 

54 

53 

53 

52 

51 

50 

49 

48 

47 

46 

44 

43 

55 

56 

55 

54 

54 

53 

52 

51 

50 

49- 

48 

47 

45 

44 

56 

57. 

56 

55 

55 

54 

53 

52 

5* 

50 

49 

48 

47 

46 

57 

58 

57 

56 

56 

55 

54 

53 

52 

51 

50 

49 

48 

47 

58 

59 

58 

57 

57 

56 

55 

54 

53 

52 

51 

50 

49 

48 

59 

60 

59 

58 

58 

57 

56 

55 

54 

53 

52 

51 

50 

49 

60 

61 

60 

59 

59 

58 

57 

56 

55 

54 

53 

52 

51 

50 

61 

62 

61 

60 

60 

59 

58 

57 

56 

55 

54 

53 

52 

52 

62 

63 

62 

61 

61 

60 

59 

58 

57 

56 

55 

55 

54 

53 

63 

64 

63 

62 

62 

61 

60 

59 

58 

57 

56 

56 

55 

54. 

64 

65 

64 

63 

63 

62 

61 

60 

59 

59 

58 

57 

56 

55 

65 

66 

65 

64 

64 

63 

62 

61 

60 

60 

59 

58 

57 

56 

66 

67 

67 

66 

65 

64 

63 

62 

61 

61 

60 

59 

58 

57 

67 

68 

68 

67 

66 

65 

64 

63 

62 

62 

61 

60 

59 

58 

68 

69, 

69 

68 

67 

66 

65 

64 

63 

63 

62 

61 

60 

59 

69 

70 

70 

69 

68 

67 

67 

66 

65 

64 

63 

62 

61 

61 

70 

71 

71 

70 

69 

68 

68 

67 

66 

65 

64 

63 

62 

62 

71 

72 

72 

71 

70 

69 

69 

68 

67 

66 

65 

64 

63 

63 

72 

73 

73 

72 

71 

70 

70 

69 

68 

67 

66 

66 

65 

64 

73 

74 

74 

73 

72 

71 

71 

70 

69 

68 

67 

67 

66 

65 

74 

75 

75 

74 

73 

72 

72 

71 

70 

69 

68 

68 

67 

66 

75 

76 

76 

75 

74 

73 

73 

72 

71 

70 

69 

69 

68 

67 

76 

77 

77 

76 

75 

74 

74 

73 

72 

71 

70 

70 

69 

68 

77 

78 

78 

77 

76 

75 

75 

74 

73 

72 

71 

71 

70 

69 

78 

79 

79 

78 

77 

76 

76 

75 

74 

73 

72 

72 

71 

70 

79 

80 

80 

79 

78 

77 

77 

76 

75 

74 

73 

73 

72 

72 

80 

t. 

00.5 

lo.O 

lo.5 

2°.0 

20.5 

3°.0 

3°.5 

4°.0 

40.6 

5°.0 

6°.5 

6o.O 

t. 

148 


TABLE  I.  —  TEMPERATURE  OF  THE  DEW-POINT,  IN  DEGREES  FAHRENHEIT. 


1 

1 

Difference  between  the  dry  and  wet  thermometers  (t  —  t"). 

J 
J 

6°.0 

60.6 

70.0 

70.5 

80.0 

8o.5 

9o.O 

90.5 

lOo.O 

100.5 

llo.O 

110.6 

120.0 

19 

-25 

19 

20 

-19 

—32 

20 

21 

-15 

-24 

-47 

21 

22 

-11 

-19 

—31 

22 

23 

—  8 

-14 

-24 

-45 

23 

24 

—  5 

-10 

-18 

-30 

24 

25 

-  2 

-  7 

—  13 

-22 

—42 

25 

26 

0 

—  4 

-  9 

-17 

-28 

26 

27 

+  3 

_  1 

-  6 

-12 

-20 

-37 

27 

28 

5 

+  1 

-  3 

—  8 

-15 

-25 

-54 

28 

29 

7 

4 

0 

-  4 

—  10 

-18 

-32 

29 

30 

9 

6 

+  2 

-  2 

—  6 

-13 

-22 

-43 

30 

31 

11 

8 

5 

-t-  1 

-  3 

—  8 

-15 

-27 

31 

32 

13 

10 

7 

4 

0 

-  4 

-10 

-18 

-33 

32 

33 

14 

12 

9 

6 

+  3 

—  1 

-   6 

-12 

-22 

-44 

33 

34 

16 

14 

11 

8 

6 

+  2 

-  2 

—  8 

-15 

-27 

34 

35 

18 

15 

13 

10 

8 

5 

+  1 

—  4 

-  9 

-18 

-32 

35 

36 
37 

38 

19 
21 

22 

17 
19 
20 

15 
17 
19 

12 
14 
16 

10 
12 
14 

8 
9 
11 

4 
6 
9 

0 
+  3 
6 

-  5 
-  2 

+  2 

-12 
-  6 

-  2 

-20 
-14 
-  8 

-42 
-25 
-16 

-52 

-29 

36 
37 
38 

39 

24 

22 

20 

18 

16 

14 

11 

8 

5 

+  1 

-  4 

—  10 

-18 

39 

40 
41 
42 
43 
44 

25 
26 
27 
29 
30 

23 
25 
26 
27 
28 

22 
23 
24 
26 
27 

20 
21 
23 
24 
26 

18 
20 
21 
23 
24 

16 
17 
19 
21 

22 

13 
15 
18 
19 
20 

11 
13 
15 

17 

18 

8 
10 
12 
14 
16 

4 
7 
10 
12 
14 

0 
+  4 
7 
9 
12 

-  5 
-  1 
+  3 
6 
9 

-12 
-  6 

+  2 
6 

40 
41 
42 
43 
44 

45 
46 
47 

48 
49 

31 
32 
33 
35 
36 

30 
31 
32 
33 
34 

28 
30 
31 
32 
33 

27 
28 
29 
30 
32 

25 
27 
28 
29 
31 

24 
25 
26 

28 
29 

22 
24 

25 
26 

28 

20 
22 
23 
25 
26 

18 
20 
22 
23 
25 

16 
18 
20 
21 

23 

13 
16 
18 
20 
21 

11 
13 
15 

17 
19 

8 
11 
13 
15 
17 

45 

46 
47 

48 
49 

50 
61 
52 
53 
54 

37 
38 
40 
41 

42 

35 
37 
38 
39 
41 

34 
36 
37 

38 

40 

33 
34 
36 
37 
39 

32 
33 
34 
36 
37 

31 
32 
33 
34 
36 

29 
31 
32 
33 
34 

28 
29 
30 
32 
33 

26 
28 
29 
30 
32 

24 
26 
28 

23 

24 
26 

28 
29 

21 

22 
24 
26 

27 

19 
21 
23 
24 
26 

50 
51 
52 
53 
54 

55 
56 
57 

58 
59 

43 
44 
46 

47 
48 

42 
43 
45 
46 
47 

41 
42 
44 
45 

46 

40 
41 

42 
44 
45 

39 
40 
41 
42 
44 

37 
39 
40 
41 
43 

36 
37 
39 
40 
41 

34 
36 

37 
39 
40 

33 
34 
36 

37 
39 

32 
33 

35 
36 
38 

30 
32 
33 
35 
36 

29 
30 
32 
33 
.35 

28 
29 
30 
32 
33 

55 
56 
57 

58 
59 

60 
61 
62 
63 
64 

49 
50 
52 
53 
54 

48 
49 
51 
52 
53 

47 
48 
50 
51 
52 

46 
47 
49 
50 
51 

45 
46 
48 
49, 
50 

44 
45 
47 
48 
49 

43 
44 
45 
47 
48 

41 

43 
44 
45 

47 

40 
42 
43 
44 
46 

39 
41 
42 
43 
45 

38 
39 
41 
42 
43 

36 
38 
39 
41 
42 

35 
36 
38 
39 
41 

60 
61 

It 

64 

65 
66 
67 

68 
69 

55 
56 
57 
58 
59 

54 
55 
56 

57 
58 

53 
54 
55 
57 
58 

52 
53 
55 
56 
57 

51 
52 
54 
55 
56 

50 
51 
53 
54 

55 

49 
50 
52 
53 
54 

48 
49 
51 
52 
53 

47 
48 
50 
51 
52 

46 
47 
48 
50 
51 

45. 
46 
47 
49 
50 

43 
45 
46 
47 

49 

42 
44 
45 

46 

48 

65 

66 
67 
68 
69 

70 
71 
72 
73 
74 

61 
62 
63 
64 
65 

60 
61 
62 
63 
64 

59 
60 
61 
62 
63 

58 
59 
60 
62 
63 

57 
58 
59 
61 
62 

56 
57 
59 
60 
61 

55 
56 
58 
59 
60 

54 
55 
57 
58 
59 

53 
55 
56 
57 

58 

52 
54 
55 
56 
57 

51 
53 
54 
55 
56 

50 
52 
53 
54 
55 

49 
51 
52 
53 
54 

70 
71 
72 
73 
74 

75 
76 

77 
78 
79 

80 

66 
67 
68 
69 
70 

72 

65 
66 
67 
68 
69 

71 

64 
65 
67 
68 
69 

70 

64 
65 
66 

67 
68 

69 

63 
64 
65 
66 
67 

68 

62 
63 
64 
66 

67 

68 

61 
62 
63 
65 
66 

67 

60 
61 
62 
64 
65 

66 

59 
61 
62 
63 
64 

65 

58 
60 
61 
62 
63 

64 

57 
59 
60 
61 
62 

63 

56 
58 
59 
60 
61 

62 

56 
57 
58 
59 
61 

62 

75 
76 

77 
78 
79 

80 

t. 

60.0 

60.5 

70.0 

70.5 

8°.0 

8°.5 

90.0 

9°.5 

100. 

100.5 

^ 

110.5 

120.0 

t. 

149 


150 


OBSERVATIONAL   METEOROLOGY. 


TABLE  I.  —  TEMPERATURE  OF  THE  DEW-POINT,  IN  DEGREES  FAHRENHEIT. 


1 

Difference  between  the  dry  and  wet  thermometers  (t  —  t"). 

J 

a 

120.0 

120.5 

13°.0 

13°.6 

140.0 

14°.6 

15°.0 

150.5 

160.0 

16°.5 

170.0 

170.5 

18°.0 

~£ 

Q 

40 

—12 

—22 

—44 

40 

41 

—  6 

—13 

—25 

41 

42 

—  2 

—  7 

-15 

-28 

42 

43 

+  2 

—  3 

-  8 

-17 

-33 

43 

44 

6 

+   1 

-  4 

-10 

—19 

—40 

44 

45 

8 

5 

0 

—  4 

-11 

-22 

-48 

45 

46 

11 

8 

+   4 

0 

-  5 

-13 

—24 

46 

47 

13 

10 

7 

+   3 

-   1 

-  6 

-14 

—27 

47 

48 

15 

12 

10 

6 

+   2 

-  2 

-  8 

-16 

-30 

48 

49 

17 

14 

12 

9 

6 

+   2 

-  3 

—  9 

-18 

-35 

49 

50 

19 

16 

14 

12 

9 

5 

+   1 

A 

-10 

-20 

-42 

50 

51 

21 

18 

17 

14 

11 

8 

5 

0 

-  5 

-12 

-22 

-52 

51 

52 

23 

21 

19 

16 

14 

11 

8 

+   4 

0 

-  6 

-13 

-25 

52 

53 

24 

22 

20 

18 

16 

14 

11 

8 

+   4 

-   1 

-   6 

-14 

-28 

53 

54 

26 

24 

22 

20 

18 

16 

13 

10 

7 

+   3 

-   2 

-  8 

—16 

54 

55 

28 

26 

24 

22 

20 

18 

16 

13 

10 

7 

+   3 

-  2 

—  8 

55 

56 

29 

27 

26 

24 

22 

20 

18 

15 

13 

10 

.     6 

+   2 

—  2 

56 

67 

30 

29 

28 

26 

24 

22 

20 

18 

15 

13 

10 

6 

+   2 

57 

58 

32 

30 

29 

27 

26 

24 

22 

20 

18 

15 

12 

9 

6 

58 

59 

33 

32 

31 

29 

27 

26 

24 

22 

20 

18 

15 

12 

9 

59 

60 

35 

33 

32 

30 

29 

27 

26 

24 

22 

20 

18 

15 

12 

60 

61 

36 

35 

33 

32 

31 

29 

28 

26 

24 

22 

20 

18 

15 

61 

62 

38 

37 

35 

34 

32 

31 

29 

28 

26 

24 

22 

20 

18 

62 

63 

39 

38 

37 

35 

34 

32 

31 

29 

28 

26 

24 

22 

20 

63 

64 

41 

39 

38 

37 

35 

34 

32 

31 

29 

28 

26 

24 

22 

64 

65 

42 

41 

40 

38 

37 

35 

34 

32 

31 

29 

28 

26 

24 

65 

66 

44 

43 

41 

40 

38 

37 

35 

34 

32 

31 

30 

28 

26 

66 

67 

45 

44 

43 

41 

40 

39 

37 

36 

34 

32 

31 

30 

28 

67 

68 

46 

45 

44 

43 

42 

40 

39 

38 

36 

34 

33 

31 

30 

68 

69 

48 

47 

46 

45 

43 

42 

40 

39 

38 

36 

34 

33 

32 

69 

70 

49 

48 

47 

46 

45 

43 

42 

41 

39 

38 

36 

35 

33 

70 

71 

51 

49 

48 

47 

46 

45 

43 

42 

41 

39 

38 

36 

35 

71 

72 

52 

51 

50 

49 

47 

46 

45 

44 

43 

41 

40 

38 

37 

72 

73 

53 

52 

51 

50 

49 

48 

46 

45 

44 

43 

41 

40 

38 

73 

74 

54 

53 

52 

51 

50 

49 

48 

47 

45 

44 

43 

41 

40 

74 

75 

56 

55 

54 

53 

52 

50 

49 

48 

47 

45 

44 

43 

42 

75 

76 

57 

56 

55 

54 

53 

52 

50 

49 

48 

47 

46 

45 

43 

76 

77 

58 

57 

56 

55 

54 

53 

52 

51 

50 

49 

48 

46 

45 

77 

78 

59 

58 

57 

56 

55 

54 

53 

52 

51 

50 

49 

48 

47 

78 

79 

61 

60 

59 

58 

57 

56 

55 

54 

53 

52 

51 

49 

48 

79 

80 

62 

61 

60 

59 

58 

57 

56 

55 

54 

53 

52 

51 

50 

80 

t. 

120.0 

120.5 

130.0 

130.5 

140.0 

140.5 

150.0 

150.5 

160.0 

160.5 

170.0 

170.5 

180.0 

t. 

METEOROLOGICAL   TABLES. 


151 


TABLE  I.  —  TEMPERATURE  OF  THE  DEW-POINT,  IN  DEGREES  FAHRENHEIT. 


3> 

Difference  between  the  dry  and  wet  thermometers  (t—f). 

u 

4) 

•to  +2 

Jj 

1 

180.0 

190.0 

200.0 

210.0 

220.0 

230.0 

240.0 

250.0 

26°.0 

270.0 

280.0 

290.0 

300.0 

1 

55 
56 

-8 

o 

-19 

55 
56 

'57 

+  2 

—10 

-48 

67 

58 

0 

-  3 

-22 

59 

9 

+   1 

-12 

59 

60 

12 

5 

—  5 

-25 

60 

61 

15 

9 

0 

-14 

61 

62 

18 

12 

+   5 

-  6 

-28 

OJL 

62 

63 

20 

15 

9 

0 

-14 

63 

64 

22 

18 

12 

+   4 

-  6 

-32 

64 

65 

24 

20 

15 

9 

0 

-16 

65 

66 

26 

22 

18 

12 

+  4 

-  7 

-34 

66 

67 

28 

24 

20 

15 

9 

-  1 

-16 

67 

68 

30 

26 

23 

18 

12 

+  4 

—  7 

—37 

68 

69 

32 

28 

25 

20 

15 

8 

0 

-17 

69 

70 

33 

30 

27 

23 

19 

12 

+   5 

-  7 

-39 

70 

71 

35 

32 

29 

25 

21 

16 

9 

0 

-17 

71 

72 

37 

33 

31 

27 

23 

18 

13 

+  5 

-  6 

-39 

72 

73 

38 

35 

32 

29 

25 

21 

16 

10 

0 

-16 

73 

74 

40 

37 

34 

31 

28 

24 

19 

13 

+   6 

-  6 

-37 

74 

75 

42 

39 

36 

32 

30 

26 

22 

16 

10 

0 

-16 

76 

76 

43 

41 

38 

34 

32 

28 

24 

20 

14 

+  6 

-  6 

-34 

76 

77 

45 

42 

40 

36 

33 

30 

26 

22 

17 

11 

+   1 

-14 

77 

78 

47 

44 

41 

38 

35 

32 

28 

24 

20 

14 

7 

-  4 

-30 

78 

79 

48 

46 

43 

40 

37 

34 

31 

27 

23 

18 

11 

+   2 

-13 

79 

80 

50 

47 

45 

42 

39 

36 

32 

29 

25 

21 

15 

8 

-  3 

80 

t. 

180.0 

19°.0 

200.0 

21°.0 

220.0 

23°.0 

240.0 

250.0 

260.0 

270.0 

280.0 

29°.0 

300.0 

t. 

152 


OBSERVATIONAL   METEOROLOGY. 


TABLE  I.  —  TEMPERATURE  OP  THE  DEW-POINT,  IN  DEGREES  FAHRENHEIT. 


1 

Difference  between  the  dry  and  wet  thermometers  (t—f). 

|4 

V 

N 

^ 

| 

lo.O 

20.0 

3°.0 

4°.0 

5°.0 

60.0 

70.0 

80.0 

9°.0 

100.0 

llo.O 

12°.0 

P 

80 

79 

77 

76 

74 

73 

72 

70 

68 

67 

65 

63 

62 

80 

81 

80 

78 

77 

75 

74 

73 

71 

70 

68 

66 

65 

63 

81 

82 

81 

79 

78 

77 

75 

74 

72 

71 

69 

68 

66 

64 

82 

83 

82 

80 

79 

78 

76 

75 

73 

72 

70 

69 

67 

65 

83 

84 

83 

81 

80 

79 

77 

76 

74 

73 

71 

70 

68 

67 

84 

85 

84 

82 

81 

80 

78 

77 

75 

74 

72 

71 

69 

68 

85 

86 

85 

83 

82 

81 

79 

78 

76 

75 

73 

72 

71 

69 

86 

87 

86 

84 

83 

82 

80 

79 

78 

76 

74 

73 

72 

70 

87 

88 

87 

85 

84 

83 

81 

80 

79 

77 

75 

74 

73 

71 

88 

89 

88 

86 

85 

84 

82 

81 

80 

78 

76 

76 

74 

72 

89 

90 

89 

87 

86 

85 

84 

82 

81 

79 

78 

77 

75 

74 

90 

91 

90 

88 

87 

86 

85 

83 

82 

80 

79 

78 

76 

75 

91 

92 

91 

89 

88 

87 

86 

84 

83 

82 

80 

79 

77 

76 

92 

93 

92 

91 

89 

88 

87 

85 

84 

83 

81 

80 

78 

77 

93 

94 

93 

92 

90 

89 

88 

86 

85 

84 

82 

81 

80 

78 

94 

95 

94 

93 

91 

90 

89 

87 

86 

85 

83 

82 

81 

79 

95 

96 

95 

94 

92 

91 

90 

88 

87 

86 

84 

83 

82 

80 

96 

97 

96 

95 

93 

92 

91 

90 

88 

87 

86 

84 

83 

81 

97 

98 

97 

96 

94 

93 

92 

91 

89 

88 

87 

85 

84 

83 

98 

99 

98 

97 

95 

94 

93 

92 

90 

89 

88 

86 

85 

84 

99 

100 

99 

98 

96 

95 

94 

93 

91 

90 

89 

87 

86 

85 

100 

101 

100 

99 

97 

96 

95 

94 

92 

91 

90 

88 

87 

86 

101 

102 

101 

100 

98 

97 

96 

95 

93 

92 

91 

90 

88 

87 

102 

103 

102 

101 

99 

98 

97 

96 

94 

93 

92 

91 

89 

88 

103 

104 

103 

102 

100 

99 

98 

97 

96 

94 

93 

92 

90 

89 

104 

105 

104 

103 

101 

100 

99 

98 

97 

95 

94 

93 

91 

90 

105 

106 

105 

104 

102 

101 

100 

99 

98 

96 

95 

94 

93 

91 

106 

107 

106 

105 

103 

102 

101 

100 

99 

97 

96 

95 

94 

92 

107 

108 

107 

106 

104 

103 

102 

101 

100 

98 

97 

96 

95 

93 

108 

109 

108 

107 

105 

104 

103 

102 

101 

99 

98 

97 

96 

94 

109 

110 

109 

108 

107 

105 

104 

103 

102 

101 

99 

98 

97 

96 

110 

t. 

,0.0 

20.0 

30.0 

40.0 

50.0 

60.0 

7°.0 

8°.0 

9°.0 

100.0 

1  1*^.0 

120.0 

t. 

METEOROLOGICAL   TABLES. 


153 


TABLE  I.  —  TEMPERATURE  OF  THE  DEW-POINT,  IN  DEGREES  FAHRENHEIT. 


f»3 
| 

Difference  between  the  dry  and  wet  thermometers  (t  —  t'). 

t 
(Dry  ther.)  | 

12°.0 

13°.0 

140.0 

150.0 

160.0 

17°.0 

180.0 

190.0 

200.0 

210.0 

220.0 

230.0 

2*0.0 

80 

62 

60 

58 

56 

54 

52 

50 

47 

45 

42 

39 

36 

32 

80 

81 

63 

61 

59 

57 

55 

53 

51 

49 

47 

44 

41 

38 

35 

81 

82 

64 

62 

61 

59 

57 

55 

53 

50 

48 

45 

43 

40 

37 

82 

83 

65 

64 

62 

60 

58 

56 

54 

52 

50 

47 

44 

42 

39 

83 

84 

67 

65 

63 

61 

59 

57 

55 

53 

51 

49 

46 

43 

41 

84 

85 

68 

66 

64 

62 

61 

59 

57 

55 

53 

50 

48 

45 

42 

85 

86 

69 

67 

66 

64 

62 

60 

58 

56 

54 

52 

49 

47 

44 

86 

87 

70 

68 

67 

65 

63 

61 

59 

57 

55 

53 

51 

48 

46 

87 

88 

71 

70 

68 

66 

64 

63 

61 

59 

57 

55 

53 

50 

48 

88 

89 

72 

71 

69 

67 

66 

64 

62 

60 

58 

56 

54 

52 

49 

89 

90 

74 

72 

70 

69 

67 

65 

63 

62 

60 

58 

56 

53 

51 

90 

91 

75 

73 

72 

70 

68 

67 

65 

63 

61 

59 

57 

55 

53 

91 

92 

76 

74 

73 

71 

69 

68 

66 

64 

62 

60 

58 

56 

54 

92 

93 

77 

75 

74 

72 

71 

69 

67 

66 

64 

62 

60 

58 

56 

93 

94 

78 

77 

75 

73 

72 

70 

69 

67 

65 

63 

61 

59 

57 

94 

95 

79 

78 

76 

75 

73 

71 

70 

68 

66 

64 

63 

61 

59 

95 

96 

80 

79 

77 

76 

74 

73 

71 

69 

68 

66 

64 

62 

60 

96 

97 

81 

80 

78 

77 

75 

74 

72 

71 

69 

67 

65 

63 

61 

97 

98 

83 

81 

80 

78 

77 

75 

73 

72 

70 

68 

67 

65 

63 

98 

99 

84 

82 

81 

79 

78 

76 

75 

73 

71 

70 

68 

66 

64 

99 

100 

85 

83 

82 

80 

79 

77 

76 

74 

73 

71 

69 

67 

66 

100 

101 

86 

84 

83 

82 

80 

79 

77 

75 

74 

72 

71 

69 

67 

101 

102 

87 

85 

84 

83 

81 

80 

78 

77 

75 

73 

72 

70 

68 

102 

103 

88 

87 

85 

84 

82 

81 

79 

78 

76 

75 

73 

71 

70 

103 

104 

89 

88 

86 

85 

83 

82 

81 

79 

78 

76 

74 

73 

71 

104 

105 
106 
107 
108 
109 

90 
91 
92 
93 
94 

89 
90 
91 
92 
93 

87 
89 
90 
91 
92 

86 

87 
88 
89 
90 

85 
86 

87 
88 
89 

83 
84 

85 
87 
88 

82 
83 
84 
85 
86 

80 
81 
83 
84 

85 

79 
80 
81 
82 
83 

77 
78 
80 
81 
82 

76 

77 
78 
79 
80 

74 

75 
76 
78 
79 

72 
74 
75 
76 

77 

105 
106 
107 
108 
109 

110 

96 

94 

93 

92 

90 

89 

87 

86 

85 

83 

82 

80 

79 

110 

t. 

120.0 

13°.0 

14°.0 

150.0 

16°.0 

170.0 

18°.0 

190.0 

200.0 

210.0 

220.0 

230.0 

24°.0 

t. 

154 


OBSERVATIONAL   METEOROLOGY. 


TABLE  I.  —  TEMPERATURE  OF  THE  DEW-POINT,  IN  DEGREES  FAHRENHEIT. 


Lg 

S 

Difference  between  the  dry  and  wet  thermometers  (t—  t'). 

oJ 

£ 

240.0 

250.0 

260.0 

270.0 

280.0 

290.0 

300.0 

310.0 

32°.0 

330.0 

340.0 

350.0 

36°.0 

1 

80 

32 

29 

25 

21 

15 

8 

-3 

-27 

80 

81 

35 

31 

28 

24 

18 

12 

+  3 

-11 

81 

82 

37 

33 

30 

26 

22 

16 

9 

-  2 

-24 

82 

83 

39 

35 

32 

28 

24 

19 

13 

+  5 

-  9 

83 

84 

41 

37 

34 

30 

27 

22 

17 

10 

0 

-20 

84 

85 

42 

39 

36 

32 

29 

25 

20 

14 

-f  6 

IT 

-54 

85 

86 

44 

41 

38 

35 

31 

28 

23 

18 

11 

+  1 

-17 

86 

87 

46 

43 

40 

37 

33 

30 

26 

21 

15 

7 

-  5 

-38 

87 

88 

48 

45 

42 

39 

35 

32 

28 

24 

19 

12 

+  3 

-13 

88 

89 

49 

47 

44 

41 

38 

34 

31 

27 

22 

16 

9 

2 

-28 

89 

90 

51 

48 

46 

43 

40 

36 

32 

29 

25 

20 

13 

+  4 

-10 

90 

91 

53 

50 

47 

45 

42 

38 

35 

32 

28 

23 

18 

10 

0 

91 

92 

54 

52 

49 

46 

44 

41 

37 

34 

30 

26 

21 

15 

+  7 

92 

93 

56 

53 

51 

48 

46 

43 

39 

36 

32 

29 

24 

19 

12 

93 

94 

57 

55 

53 

50 

47 

45 

42 

38 

35 

31 

27 

22 

16 

94 

95 

59 

56 

54 

52 

49 

46 

44 

40 

37 

33 

30 

25 

20 

95 

96 

60 

58 

56 

53 

51 

48 

46 

43 

39 

36 

32 

28 

24 

96 

97 

61 

59 

57 

55 

53 

50 

47 

45 

41 

38 

34 

31 

26 

97 

98 

63 

61 

59 

57 

54 

52 

49 

47 

44 

40 

37 

33 

29 

98 

99 

64 

62 

60 

58 

56 

54 

51 

48 

46 

43 

39 

35 

32 

99 

100 

66 

64 

62 

60 

57 

55 

53 

50 

48 

45 

41 

38 

34 

100 

101 

67 

65 

63 

61 

59 

57 

54 

52 

49 

47 

44 

40 

37  ' 

101 

102 

68 

66 

65 

63 

61 

58 

56 

54 

51 

49 

46 

43 

39 

102 

103 

70 

68 

66 

64 

62 

60 

58 

55 

53 

50 

48 

45 

41 

103 

104 

71 

69 

67 

65 

63 

61 

59 

57 

55 

52 

50 

47 

44 

104 

105 

72 

70 

69 

67 

65 

63 

61 

59 

56 

54 

52 

49 

46 

105 

106 

74 

72 

70 

68 

66 

64 

62 

60 

58 

56 

53 

51 

48 

106 

107 

75 

73 

71 

70 

68 

66 

64 

62 

60 

57 

55 

52 

50 

107 

108 

76 

74 

73 

71 

69 

67 

65 

63 

61 

59 

57 

54 

52 

108 

109 

77 

76 

74 

72 

71 

69 

67 

65 

63 

61 

58 

56 

54 

109 

110 

79 

77 

75 

74 

72 

70 

68 

66 

64 

62 

60 

58 

55 

110 

t. 

240.0 

250.0 

260.0 

270.0 

280.0 

290.0 

300.0 

310.0 

320.0 

330.0 

340.0 

350.0 

36°.0 

'• 

METEOROLOGICAL   TABLES. 


155 


TABLE  I.  —  TEMPERATURE  OF  THE  DEW-POINT,  IN  DEGREES  FAHRENHEIT. 


J 

Difference  between  the  dry  and  wet  thermometers  (t—f). 

1 

>> 
b 

36°.0 

370.0 

380.0 

390.0 

400.0 

410.0 

420.0 

43°.0 

440.0 

450.0 

460.0 

470.0 

480.0 

£ 

e 

89 

—28 

S!) 

90 
91 

—10 
0 

—22 

90 

Q  1 

92 

+  7 

-  7 

-16 

•  '  1 

'.1  " 

93 

12 

+  2 

—  4 

93 

94 

16 

8 

+  4 

—37 

94 

95 

20 

13 

10 

-12 

95 

96 

24 

15 

15 

-  1 

—25 

96 

97 

26 

21 

19 

+  7 

-  8 

97 

98 

29 

25 

23 

12 

+  2 

-18 

98 

99 

32 

28 

26 

17 

9 

-  4 

-42 

99 

100 

34 

30 

29 

21 

14 

+  5 

-12 

100 

101 

37 

32 

32 

24 

18 

11 

0 

—25 

|01 

102 

39 

35 

34 

27 

22 

16 

+  7 

-  7 

102 

103 

41 

38 

34 

30 

26 

20 

13 

+  3 

—16 

103 

104 

44 

40 

37 

32 

29 

24 

18 

10 

-  2 

-38 

104 

105 

46 

43 

39 

35 

31 

27 

22 

15 

+  6 

-10 

105 

106 

48 

45 

42 

38 

34 

30 

25 

20 

12 

+  1 

-22 

106 

107 

50 

47 

44 

40 

37 

32 

28 

24 

17 

9 

—  5 

107 

108 

52 

49 

46 

43 

39 

35 

31 

27 

21 

14 

+  5 

-13 

108 

109 

54 

51 

48 

45 

42 

38 

34 

30 

25 

19 

12 

0 

-28 

109 

110 

55 

53 

50 

47 

44 

41 

37 

33 

28 

23 

17 

+  8 

-  7 

110 

t. 

36°.0 

370.0 

380.0 

390.0 

400.0 

410.0 

420.0 

430.0 

440.0 

450.0 

46°.0 

470.0 

480.0 

t. 

TABLE  II.  —  RELATIVE  HUMIDITY,  PER  CENT. 


SH 

Difference  between  the  dry  and  wet  thermometers  (t  —  f). 

h 

2 

| 

002 

00.4 

00.6 

00.8 

lo.O 

10.2 

lo.4 

lo.6 

lo.8 

20.0 

2°.2 

2°.4 

2°.6 

| 

-40 

46 

-40 

-39 

49 

-39 

-38 

51 

-38 

-37 

54 

-37 

-36 

56 

-36 

-35 

59 

-35 

-34 

61 

22 

-34 

-33 

63 

25 

-33 

-32 

65 

30 

-32 

—31 

67 

34 

-31 

—30 

69 

38 

-30 

—29 

71 

42 

-29 

-28 

72 

45 

17 

-28 

—  27 

74 

48 

22 

-27 

-26 

76 

51 

26 

-26 

-25 

77 

53 

31 

—  25 

-24 

78 

56 

34 

12 

-24 

-23 

79 

58 

37 

16 

-23 

-22 

80 

60 

40 

20 

-22 

—  21 

81 

62 

44 

25 

-21 

-20 

82 

64 

47 

29 

11 

-20 

-19 

83 

66 

49 

33 

16 

-19 

—  18 

84 

68 

52 

36 

20 

-18 

-17 

85 

70 

54 

39 

24 

9 

-17 

-16 

86 

71 

57 

43 

28 

14 

-16 

—  15 

86 

73 

59 

46 

32 

19 

-15 

-14 

87 

74 

61 

48 

36 

23 

10 

—  14 

-13 

88 

76 

63 

51 

39 

27 

15 

-13 

-12 

88 

77 

65 

53 

42 

30 

19 

7 

-12 

-11 

89 

78 

67 

56 

45 

34 

23 

12 

—  11 

-10 

90 

79 

68 

58 

48 

37 

26 

16 

5 

-10 

—  9 

90 

80 

70 

60 

50 

40 

30 

20 

10 

-  9 

-  8 

90 

81 

71 

62 

52 

43 

33 

24 

14 

5 

-  8 

-  7 

91 

82 

73 

63 

54 

45 

36 

27 

18 

9 

— 

-  6 

91 

83 

74 

65 

56 

48 

39 

31 

22 

13 

5 

— 

-  5 

92 

83 

75 

67 

58 

50 

42 

34 

25 

17 

9 



4 

92 

84 

76 

68 

60 

52 

45 

37 

29 

21 

13 

5 

— 

-  3 

92 

85 

77 

70 

62 

55 

47 

40 

32 

25 

17 

10 

j 

—  2 

93 

86 

78 

71 

64 

57 

•  50 

42 

35 

28 

21 

14 

7 

-  2 

-  1 

93 

86 

79 

72 

66 

59 

52 

45 

38 

31 

25 

18 

11 

-  1 

0 

93 

87 

80 

74 

67 

61 

54 

48 

41 

35 

28 

22 

15 

0 

+  1 

94 

87 

81 

75 

69 

63 

56 

50 

44 

38 

32 

25 

19 

+  1 

2 

94 

88 

82 

76 

70 

64 

58 

52 

46 

40 

35 

29 

23 

2 

3 

94 

88 

83 

77 

71 

66 

60 

54 

49 

43 

37 

32 

26 

3 

4 

94 

89 

83 

78 

73 

67 

62 

56 

51 

45 

40 

34 

29 

4 

5 

95 

89 

84 

79 

74 

68 

63 

58 

53 

48 

42 

37 

32 

5 

6 

95 

90 

85 

80 

75 

70 

65 

60 

54 

50 

44 

39 

34 

6 

7 

95 

90 

85 

80 

76 

71 

66 

61 

56 

51 

47 

42 

37 

7 

8 

95 

91 

86 

81 

76 

72 

67 

62 

58 

53 

49 

44 

39 

8 

9 

96 

91 

86 

82 

77 

73 

68 

64 

59 

55 

51 

46 

42 

9 

10 

96 

91 

87 

83 

78 

74 

69 

65 

61 

57 

52 

48 

44 

10 

11 

96 

92 

87 

83 

79 

75 

71 

66 

62 

58 

54 

50 

46 

11 

12 

96 

92 

88 

84 

80 

76 

72 

68 

64 

60 

56 

52 

48 

12 

13 

96 

92 

88 

84 

81 

77 

73 

69 

65 

61 

58 

54 

50 

13 

14 

96 

93 

89 

85 

81 

78 

74 

70 

67 

63 

59 

56 

52 

14 

15 

96 

93 

89 

86 

82 

79 

75 

71 

68 

64 

61 

57 

54 

15 

16 

97 

93 

90 

86 

83 

79 

76 

73 

69 

66 

62 

59 

56 

16 

17 

97 

93 

90 

87 

83 

80 

77 

74 

70 

67 

64 

60 

57 

17 

18 

97 

94 

90 

87 

84 

81 

78 

74 

71 

68 

65 

62 

59 

18 

19 

97 

94 

91 

88 

84 

81 

78 

75 

72 

69 

66 

63 

60 

19 

20 

97 

94 

91 

88 

85 

82 

79 

76 

73 

70 

67 

64 

61 

20 

t. 

00.2 

00.4 

00.6 

00.8 

10.0 

10.2 

10.4 

10.6 

10.8 

20.0 

2°.2 

20.4 

2°.6 

t. 

156 


METEOROLOGICAL   TABLES. 


157 


TABLE  II RELATIVE  HUMIDITY,  PER  CENT. 


I 

Difference  between  the  dry  and  wet  thermometers  (t  —  t'). 

I 

j 

20.6 

20.8 

30.0 

8°.2 

30.4 

30.6 

30.8 

4o.O 

40.2 

4o.4 

"!  r 

—  2 

7 

—  2 

-  1 

11 

4 

-  1 

0 

15 

9 

o 

+  1 

19 

13 

7 

-I-  i 

2 

23 

17 

11 

5 

2 

3 

26 

20 

15 

9 

4 

3 

4 

29 

24 

18 

13 

8 

2 

4 

5 

32 

27 

22 

16 

11 

6 

5 

6 

34 

29 

25 

20 

15 

10 

5 

6 

7 

37 

32 

28 

23 

18 

13 

9 

4 

7 

8 

39 

35 

30 

26 

21 

17 

12 

8 

3 

8 

9 

42 

37 

33 

28 

24 

20 

15 

11 

7 

2 

9 

10 

44 

40 

35 

31 

27 

23 

19 

14 

10 

6 

10 

11 

46 

42 

38 

34 

30 

26 

22 

18 

14 

10 

11 

12 

48 

44 

40 

36 

32 

28 

25 

21 

17 

13 

12 

13 

50 

46 

42 

39 

35 

31 

27 

24 

20 

16 

13 

14 

52 

48 

45 

41 

37 

34 

30 

27 

23 

19 

14 

15 

54 

50 

47 

43 

40 

36 

33 

29 

26 

23 

15 

16 

56 

52 

49 

46 

42 

39 

36 

32 

29 

25 

16 

17 

57 

54 

51 

48 

44 

41 

38 

35 

31 

28 

17 

18 

59 

56 

53 

49 

46 

43 

40 

37 

34 

31 

18 

19 

60 

57 

54 

51 

48 

45 

42 

39 

36 

33 

19 

20 

61 

58 

56 

53 

50 

47 

44 

41 

38 

35 

20 

t. 

20.6 

20.8 

30.0 

30.2 

3°.4 

30.6 

30.8 

40.0 

40.3 

40.4 

t. 

t. 

40.6 

4o.8 

5o.O 

5°.2 

6°.4 

5°.6 

50.8 

6°.0 

6o.2 

6°.4 

t. 

8 
9 

8 
9 

10 

2 

10 

11 

6 

2 

11 

12 

9 

5 

2 

12 

13 

13 

9 

5 

2 

13 

14 

16 

12 

9 

5 

.2 

14 

15 

19 

16 

12 

9 

5 

2 

15 

16 

22 

19 

16 

12 

9 

6 

2 

16 

17 

25 

22 

19 

16 

12 

9 

6 

3 

17 

18 
19 

28 
30 

25 
27 

22 
24 

19 
21 

16 
19 

13 
16 

9 
13 

6 
10 

3 

7 

4 

18 
19 

20 

33 

30 

27 

24 

21 

19 

16 

13 

10 

7 

20 

t. 

40.6 

40.8 

50.0 

5°.2 

50.4 

5°.6 

50.8 

6°.0 

60.2 

60.4 

t. 

158 


TABLE  II. — RELATIVE  HUMIDITY,  PER  CENT. 


.1 

Difference  between  the  dry  and  wet  thermometers  (t—  t'). 

1 

1 

00.5 

lo.O 

10.5 

2°-0 

20.5 

30.0 

3°.5 

40.0 

40.5 

60.0 

50.5 

6o.« 

L 

20 

92 

85 

77 

70 

63 

56 

48 

41 

34 

27 

20 

13 

20 

21 

93 

85 

78 

71 

64 

57 

50 

43 

36 

29 

23 

16 

21 

22 

93 

86 

79 

72 

65 

58 

51 

45 

38 

32 

25 

19 

22 

23 

93 

86 

80 

73 

66 

60 

53 

46 

40 

34 

27 

21 

23 

24 

93 

87 

80 

74 

67 

61 

54 

48 

42 

36 

30 

24 

24 

25 

94 

87 

81 

74 

G8 

62 

56 

50 

44 

38 

32 

26 

25 

26 

94 

88 

81 

75 

69 

63 

57 

51 

45 

40 

34 

28 

26 

27 

94 

88 

82 

76 

70 

64 

59 

53 

47 

42 

36 

30 

27 

28 

94 

88 

82 

77 

71 

65 

60 

54 

49 

43 

38 

33 

28 

29 

94 

89 

83 

77 

72 

66 

61 

56 

50 

45 

40 

35 

29 

30 

94 

89 

84 

78 

73 

67 

62 

57 

52 

47 

41 

36 

30 

31 

95 

89 

84 

79 

74 

68 

'63 

58 

53 

48 

43 

38 

31 

32 

95 

90 

84 

79 

74 

69 

64 

r,o 

54 

50 

45 

40 

32 

33 

95 

90 

85 

80 

75 

70 

65 

GO 

56 

51 

47 

42 

33 

34 

95 

91 

86 

81 

fO 

72 

G7 

62 

57 

53 

48 

44 

34 

35 

95 

91 

86 

82 

76 

73 

69 

65 

59 

54 

50 

45 

35 

36 

96 

91 

86 

82 

77 

73 

70 

66 

61 

56 

51 

47 

36 

37 

96 

91 

87 

82 

78 

74 

70 

66 

62 

57 

52 

48 

37 

38 

96 

92 

87 

83 

79 

75 

71 

67 

63 

58 

54 

50 

38 

39 

96 

92 

88 

83 

79 

75 

72 

68 

63 

59 

55 

52 

39 

40 

96 

92 

88 

84 

80 

76 

72 

68 

64 

GO 

56 

53 

40 

41 

96 

92 

88 

84 

80 

76 

72 

69 

65 

61 

57 

54 

41 

42 

96 

92 

88 

84 

81 

77 

73 

69 

65 

62 

58 

55 

42 

43 

96 

92 

88 

85 

81 

77 

74 

70 

66 

63 

59 

56 

43 

44 

96 

92 

88 

85 

81 

78 

74 

70 

67 

63 

60 

57 

44 

45 

96 

92 

89 

86 

82 

78 

75 

71 

67 

64 

61 

58 

45 

46 

96 

93 

89 

85 

82 

79 

75 

72 

68 

65 

61 

58 

46 

47 

96 

93 

89 

86 

83 

79 

76 

72 

69 

66 

62 

59 

47 

48 

96 

93 

89 

86 

83 

79 

76 

73 

69 

66 

63 

60 

48 

49 

97 

93 

90 

86 

83 

80 

76 

73 

70 

67 

63 

GO 

49 

50 

97 

93 

90 

87 

83 

80 

77 

74 

70 

67 

64 

61 

50 

51 

97 

93 

90 

87 

84 

81 

77 

74 

71 

68 

65 

62 

51 

52 

97 

94 

90 

87 

84 

81 

78 

75 

72 

69 

66. 

63 

52 

53 

97 

94 

91 

87 

84 

81 

78 

75 

72 

69 

66 

63 

53 

54 

97 

94 

91 

88 

85 

82 

79 

76 

73 

70 

67 

64 

54 

55 

97 

94 

91 

88 

85 

82 

79 

73 

70 

68 

65 

55 

56 

97 

94 

91 

88 

85 

82 

80 

74 

71 

68 

65 

56 

57 

97 

94 

91 

88 

86 

83 

80 

74 

71 

69 

66 

57 

58 

97 

94 

91 

89 

86 

83 

80 

78 

75 

72 

69 

67 

58 

59 

97 

94 

92 

89 

86 

83 

81 

7S 

75 

72 

70 

67 

59 

60 

97 

94 

92 

89 

86 

84 

81 

78 

75 

73 

70 

68 

60 

61 

97 

94 

92 

89 

87 

84 

81 

78 

76 

73 

71 

68 

61 

62 

97 

95 

92 

89 

87 

84 

81 

79 

76 

74 

71 

69 

62 

63 

97 

95 

92 

89' 

87 

84 

82 

79 

77 

74 

72 

69 

63 

64 

97 

95 

92 

90 

87 

85 

82 

79 

77 

74 

72 

70 

64 

65 

97 

95 

92 

90 

87 

85 

82 

80 

77 

75 

72 

70 

65 

66 

97 

95 

92 

90 

87 

85 

82 

80 

78 

75 

73 

71 

66 

67 

98 

95 

93 

90 

88 

85 

83 

80 

78 

76 

73 

71 

67 

68 

98 

95 

93 

90 

88 

85 

83 

81 

78 

76 

74 

71 

68 

69 

98 

95 

93 

90 

88 

86 

83 

81 

78 

76 

74 

7'2 

69 

70 

98 

95 

93 

90 

88 

86 

83 

81 

79 

77 

74 

72 

70 

71 

98 

95 

93 

91 

88 

86 

84 

81 

79 

77 

75 

72 

71 

72 

98 

95 

93 

91 

88 

86 

84 

82 

79 

77 

75 

73 

72 

73 

98 

95 

93 

91 

88 

86 

84 

82 

80 

78 

75 

73 

73 

74 

98 

95 

93 

91 

88 

86 

84 

82 

80 

78 

76 

74 

74 

75 

98 

95 

93 

91 

89 

87 

84 

82 

80 

78 

76 

74 

75 

76 

98 

95 

93 

91 

89 

87 

85 

82 

80 

78 

76 

74 

76 

77 

98 

95 

93 

91 

89 

87 

85 

83 

80 

78 

76 

74 

77 

78 

98 

96 

93 

91 

89 

87 

85 

83 

81 

79 

77 

75 

78 

79 

98 

96 

94 

91 

89 

87 

85 

83 

81 

79 

77 

75 

79 

80 

98 

96 

94 

92 

89 

87 

85 

83 

81 

79 

77 

75 

80 

t. 

00.5 

10.0 

10.5 

20.0 

20.5 

30.0 

30.5 

40.0 

40.5 

5°.0 

60.5 

60.0 

t. 

TABLE  II. — RELATIVE  HUMIDITY,  PER  CENT. 


159 


t 
(Dry  ther.) 

Difference  between  the  dry  and  wet  thermometers  (t—f). 

I  (Dry  ther.)!, 

6°.0 

6°.5 

7°.0 

7°.5 

8°.0 

So.5 

90.0 

9°.5 

10°.  0 

10°.5 

11°.0 

110.6 

120.0 

19 

10 

19 

20 

13 

6 

20 

21 

16 

9 

2 

21 

22 

19 

12 

6 

22 

23 

21 

15 

9 

2 

•2« 

24 

24 

17 

11 

6 

24 

25 

26 

20 

14 

8 

3 

25 

26 

28 

23 

17 

11 

6 

26 

27 

30 

25 

19 

14 

9 

3 

27 

28 

33 

27 

22 

17 

11 

6 

1 

28 

29 

35 

29 

24 

19 

14 

9 

4 

29 

30 

36 

31 

26 

22 

17 

12 

7 

2 

30 

31 

38 

33 

29 

24 

19 

14 

10 

5 

31 

32 

40 

35 

31 

26 

21 

17 

12 

8 

3 

32 

33 

42 

37 

33 

28 

24 

19 

15 

10 

6 

2 

33 

34 

44 

39 

35 

30 

26 

21 

17 

13 

9 

4 

34 

35 

45 

41 

37 

32 

28 

24 

19 

15 

12 

7 

3 

35 

36 

47 

43 

38 

34 

30 

26 

22 

18 

14 

10 

6 

2 

36 

37 

48 

44 

40 

36 

32 

28 

24 

20 

16 

12 

8 

5 

1 

37 

38 

50 

46 

42 

38 

34 

30 

26 

22 

18 

15 

11 

7 

3 

38 

39 

52 

48 

44 

40 

36 

32 

'  28 

24 

20 

17 

13 

9 

6 

39 

40 
41 
42 
43 
44 

53 
54 
55 
56 

57 

49 
50 
51 
52 
53 

45 
46 
48 
49 
50 

41 
43 
44 
46 

47 

38 
39 
40 
42 
43 

34 
36 
37 
38 
40 

30 
32 
34 
35 
37 

26 
29 
30 
32 
33 

22 
24 
27 
29 
30 

19 
21 
23 
25 

27 

16 
18 
20 
22 
24 

12 
14 
16 
19 
21 

8 
10 
13 
15 
17 

40 
41 
42 
43 
44 

45 
46 
47 

48 
49 

58 
58 
59 
60 
60 

54 
55 
56 
56 
57 

51 
52 
53 
53 
54 

48 
49 
50 
51 
52 

44 

46 
47 
48 
49 

41 
42 
44 
45 
46 

38 
39 
40 
42 
43 

35 
36 
38 
39 
40 

32 
33 
34 
36 
37 

29 
30 
31 
33 
34 

26 
27 
28 
30 
31 

22 
23 

25 

27 
29 

19 
21 
22 
24 
26 

45 
46 
47 

48 
49 

50 
51 
52 
S3 
54 

61 
62 
63 
63 
64 

58 
59 
60 
61 
61 

55 
56 

57 
58 
59 

52 
53 
54 
55 
56 

50 
50 
51 
52 
53 

47 

48 
48 
49 
50 

44 
45 
46 
47 
48 

41 
42 
43 
44 
45 

38 
39 
40 
42 
43 

36 
37 

38 
29 
40 

33 
34 
35 
36 
38 

30 
31 
33 
34 
35 

27 
28 
30 
31 
32 

50 
51 
52 
53 
54 

55 

56 
57 

58 
59 

65 
65 
66 
67 
67 

62 
63 
64 
64 
65 

59 
60 
61 
61 
62 

57 
57 
58 
59 
60 

54 
55 
55 
56 
57 

51 
52 
53 
53« 
54 

49 
50 
50 
51 
52 

46 
47 
48 
49 
49 

43 
44 

45 
46 
47 

41 
42 
43 
44 
45 

39 
40 
40 
42 
43 

36 
37 
38 
39 
40 

34 
35 
36 
37 
38 

55 
56 
57 

58 
59 

60 
61 
€2 
63 
64 

68 
68 
69 
69 
70 

65 
66 

66 
67 
67 

63 
63 
64 
64 
65 

60 
61 
61 
62 
62 

58 
58 
59 
60. 
60 

55 
56 
57 

57 
58 

53 
54 
54 
55 
56 

50 
51 
52 
53 
53 

48 
49 
50 
51 
51 

46 
47 

47 
48 
49 

44 
44 
45 
46 
47 

41 
42 
43 
44 
45 

39 
40 
41 
42 
43 

60 
61 
62 
63 
64 

65 
66 
67 

68 
69 

70 
71 
71 
71 

72 

68 
68 
69 
69 
70 

65 
66 
66 
67 
67 

63 
63 
64 
65 
65 

61 
61 
62 
63 
63 

59 
59 
60 
60 
61 

56 
57 
58 
58 
59 

54 
55 
55 
56 
57 

52 
53 
53 
54 
55 

50 
51 
51 
52 
53 

48 
49 
49 
50 
51 

46 
47 
47 
48 
49 

44 
45 
45 
46 
47 

65 
66- 
67 
6H 
69 

70 

7: 

7* 

7- 

72 
72 
73 
73 
74 

70 
70 
71 

71 

72 

68 
68 
69 
69 
70 

66 
66 
67 
67 

68 

64 
64 
65 
65 
66 

62 
62 
63 
63 
64 

60 
60 
61 
61 
62 

57 
58 
59 
59 
60 

55 
56 
57 
57 

58 

53 
54 
55 
55 
56- 

52 
52 
53 
53 
54 

50 
50 
51 
52 
52 

48 
48 
49 
50 
50 

70 
71 
72 
73 
74 

7o 

76 

7 
78 
79 

74 
74 
74 
75 
75 

72 
72 
73 
73 
73 

70 
70 
71 
71 
71 

68 
68 
69 
69 
70 

66 
66 
67 
67 

68 

64 
64 
65 
65 
66 

62 
63 
63 
63 
64 

60 
61 
61 
62 
62 

58 
59 
59 
60 
60 

56 

57 
57 

58 
58 

55 
55 
56 
56 

57 

53 
53 
54 
54 
55 

51 
52 
52 
53 
53 

75 
76 

77 
78 
79 

8 

75 

73 

72 

70 

68 

66 

64 

63 

61 

59 

57 

55 

54 

80 

t. 

6°.0 

60.6 

7°.0 

7°.  5 

8°.0 

80.5 

9°.0 

9°.5 

100.0 

10°. 

llo.O 

110.5 

120. 

t. 

160 


OBSERVATIONAL   METEOROLOGY. 


TABLE  II.  —  RELATIVE  HUMIDITY,  PER  CENT. 


J 

Difference  between  the  dry  and  wet  thermometers  (t—f). 

1 

b 

p 

12°.0 

120.5 

13°.0 

13°.5 

14°.0 

140.5 

150.0 

150.5 

16°.0 

160.6 

170.0 

170.5 

180.0 

1 

40 

8 

5 

! 

40 

41 

10 

7 

4 

41 

42 

13 

10 

6 

3 

42 

43 

15 

12 

9 

5 

2 

43 

44 

17 

14 

11 

8 

5 

1 

44 

45 

19 

16 

13 

10 

7 

4 

1 

45 

46 

21 

18 

15 

12 

9 

6 

3 

46 

47 

22 

20 

16 

14 

11 

8 

5 

3 

47 

48 

24 

21 

19 

16 

13 

10 

7 

5 

2 

48 

49 

26 

23 

20 

17 

15 

12 

9 

7 

4 

1 

49 

50 

27 

24 

22 

19 

16 

14 

11 

9 

6 

4 

1 

50 

51 

28 

26 

23 

21 

18 

16 

13 

10 

8 

5 

3 

51 

52 

30 

27 

24 

22 

20 

17 

15 

12 

10 

7 

5 

52 

53 

31 

29 

26 

24 

21 

19 

16 

14 

12 

9 

7 

4 

2 

53 

54 

32 

30 

28 

25 

23 

20 

18 

15 

13 

11 

8 

6 

4 

54 

55 

34 

31 

29 

26 

24 

22 

19 

17 

15 

12 

10 

8 

6 

55 

56 

35 

33 

30 

28 

25 

23 

21 

19 

16 

14 

12 

10 

8 

56 

57 

36 

34 

32 

29 

27 

24 

22 

20 

18 

16 

13 

11 

9 

57 

58 

37 

35 

33 

30 

28 

26 

24 

21 

19 

17 

15 

13 

11 

58 

59 

38 

36 

34 

31 

29 

27 

25 

23 

21 

18 

16 

14 

12 

59 

60 

39 

37 

34 

32 

30 

28 

26 

24 

22 

20 

18 

16 

14 

60 

61 

40 

38 

35 

33 

32 

.29 

27 

25 

23 

21 

19 

17 

15 

61 

62 

41 

39 

37 

34 

32 

30 

28 

26 

24 

22 

20 

18 

16 

62 

63 

42 

40 

38 

35 

33 

31 

29 

28 

26 

24 

22 

20  , 

18 

63 

64 

43 

41 

38 

36 

34 

32 

30 

29 

27 

25 

23 

21 

19 

64 

65 

44 

42 

39 

37 

35 

33 

31 

29 

28 

26 

24 

22 

20 

65 

66 

45 

42 

40 

38 

36 

34 

32 

30 

29 

27 

25 

23 

22 

66 

67 

45 

43 

41 

39 

37 

35 

33 

32 

30 

28 

26 

25 

23 

67 

68 

46 

44 

42 

40 

38 

36 

34 

33 

31 

29 

27 

26 

24 

68 

69 

47 

45 

43 

41 

39 

37 

35 

33 

32 

30 

28 

26 

25 

69 

70 

48 

46 

44 

42 

40 

38 

36 

34 

33 

31 

29 

27 

26 

70 

71 

48 

46 

45 

43 

41 

39 

37 

35 

34 

32 

30 

28 

27 

71 

72 

49 

47 

45 

43 

42 

40 

38 

36 

35 

33 

31 

30 

28 

72 

73 

50 

48 

46 

44 

42 

41 

39 

37 

35 

34 

32 

30 

29 

73 

74 

50 

48 

47 

45 

43 

41 

40 

38 

36 

35 

33 

31 

30 

74 

75 

51 

49 

47 

46 

44 

42 

40 

39 

37 

35 

34 

32 

31 

75 

76 

52 

50 

48 

46 

45 

43 

41 

39 

38 

36 

35 

33 

31 

76 

77 

52 

50 

49 

47 

45 

44 

42 

40 

39 

37 

35 

34 

32 

77 

78 

53 

51 

49 

48 

46 

44 

43 

41 

39 

38 

36 

35 

33 

78 

79 

53 

52 

50 

48 

47 

45 

43 

42 

40 

39 

37 

36 

34 

79 

80 

54 

52 

51 

49 

47 

45 

44 

42 

41 

39 

38 

36 

35 

80 

t. 

12°.0 

120.5 

130.0 

13°.5 

140.0 

140.5 

150.0 

150.5 

160.0 

160.5 

170.0 

170.5 

18°.0 

t. 

METEOROLOGICAL   TABLES. 


161 


TABLE  II.  —  RELATIVE  HUMIDITY,  PER  CENT. 


•»+» 

Difference  between  the  dry  and  wet  thermometers  (t—£). 

1 

o. 

18°.0 

190.0 

200.0 

210.0 

220.0 

230.0 

24°.0 

250.0 

260.0 

270.0 

280.0 

290.0 

300.0 

"I 

55 

KG 

6 

1 

55 

oo 
57 

9 

5 

56 

58 

11 

7 

2 

57 

- 

59 

12 

8 

4 

OS 

59 

60 

14 

10 

p 

2 

60 

61 

15 

11 

7 

3 

i;  1 

62 

16 

13 

9 

5 

1 

Oi 

63 

18 

14 

10 

7 

3 

*JjJ 

64 

19 

15 

12 

8 

5 

1 

64 

65 

20 

17 

13 

10 

6 

3 

65 

66 

22 

18 

14 

11 

8 

4 

1 

66 

67 

23 

19 

16 

12 

9 

6 

2 

67 

68 

24 

20 

17 

14 

10 

7 

4 

1 

68 

69 

25 

22 

18 

15 

12 

8 

5 

2 

69 

70 

26 

23 

19 

16 

13 

10 

7 

4 

1 

70 

71 

27 

24 

20 

17 

14 

11 

8 

5 

2 

71 

72 

28 

24 

22 

18 

15 

12 

9 

6 

3 

1 

72 

73 

29 

25 

22 

19 

16 

13 

10 

8 

5 

2 

73 

74 

30 

26 

23 

20 

18 

15 

12 

9 

6 

3 

1 

74 

75 

31 

27 

24 

21 

19 

16 

13 

10 

7 

5 

2 

75 

76 

31 

28 

25 

22 

20 

17 

14 

11 

8 

6 

3 

1 

76 

77 

32 

29 

26 

23 

20 

18 

15 

12 

10 

7 

4 

2 

77 

78 

33 

30 

27 

24 

21 

19 

16 

13 

11 

8 

6 

3 

1 

78 

79 

34 

31 

28 

25 

22 

19 

17 

14 

12 

9 

7 

4 

2 

79 

80 

35 

32 

29 

26 

23 

20 

18 

15 

13 

10 

8 

6 

3 

80 

t. 

180.0 

19°.0 

200.0 

210.0 

220.0 

2300 

240.0 

250.0 

260.0 

270.0 

280.0 

290.0 

300.0 

t. 

162 


OBSERVATIONAL   METEOROLOGY. 


TABLE  II.  —  RELATIVE  HUMIDITY,  PER  CENT. 


J 

Difference  between  the  dry  and  wet  thermometers  (t—  t'). 

J 

q 

lo.O 

2o.O 

30.0 

4o.O 

5°.0 

6°.0 

7°.0 

8°.0 

9°.0 

10°.0 

110.0 

12°.0 

| 

80 

96 

92 

87 

83 

79 

75 

72 

68 

64 

61 

57 

54 

80 

81 

96 

92 

88 

84 

80 

76 

72 

68 

65 

61 

58 

54 

81 

82 

96 

92 

88 

84 

80 

76 

72 

69 

65 

62 

58 

55 

82 

83 

96 

92 

88 

84 

80 

76 

73 

69 

66 

62 

59 

55 

83 

84 

96 

92 

88 

84 

80 

77 

73 

69 

66 

63 

59 

56 

84 

85 

96 

92 

88 

84 

80 

77 

73 

70 

66 

63 

60 

56 

85 

86 

96 

92 

88 

84 

81 

77 

73 

70 

67 

63 

60 

57 

86 

87 

96 

92 

88 

84 

81 

77 

74 

70 

67 

64 

60 

57 

87 

88 

96 

92 

88 

85 

81 

77 

74 

71 

67 

64 

61 

58 

88 

89 

96 

92 

88 

85 

81 

78 

74 

71 

68 

64 

61 

58 

89 

90 

96 

92 

88 

85 

81 

78 

75 

71 

68 

65 

62 

59 

90 

91 

96 

92 

89 

85 

82 

78 

75 

71 

68 

65 

62 

59 

91 

92 

96 

92 

89 

85 

82 

78 

75 

72 

69 

65 

62 

59 

92 

93 

96 

93 

89 

85 

82 

78 

75 

72 

69 

66 

63 

60 

93 

94 

96 

93 

89 

86 

82 

79 

75 

72 

69 

66 

63 

60 

94 

95 

96 

93 

89 

86 

82 

79 

76 

72 

69 

66 

63 

60 

95 

96 

96 

93 

89 

86 

82 

79 

76 

73 

70 

67 

64 

61 

96 

97 

96 

93 

89 

86 

82 

79 

76 

73 

70 

67 

64 

61 

97 

98 

96 

93 

89 

86 

83 

79 

76 

73 

70 

67 

64 

61 

98 

99 

96 

93 

89 

86 

83 

80 

76 

73 

70 

68 

65 

62 

99 

100 

97 

93 

90 

86 

83 

80 

77 

74 

71 

68 

65 

62 

100 

101 

97 

93 

90 

86 

83 

80 

77 

74 

71 

68 

65 

62 

101 

102 

97 

93 

90 

86 

83 

80 

77 

74 

71 

68 

-  65 

63 

102 

103 

97 

93 

90 

87 

83 

80 

77 

74 

71 

69 

66 

63 

103 

104 

97 

93 

90 

87 

83 

80 

77 

74 

72 

69 

66 

63 

104 

105 

97 

93 

90 

87 

84 

81 

78 

75 

72 

69 

66 

64 

105 

106 

97 

93 

90 

87 

84 

81 

78 

75 

72 

69 

66 

64 

106 

107 

97 

93 

90 

87 

84 

81 

78 

75 

72 

69 

67 

64 

107 

108 

97 

93 

90 

87 

84 

81 

78 

75 

72 

70 

67 

64 

108 

109 

97 

93 

90 

87 

84 

81 

78 

75 

73 

70 

67 

65 

109 

110 

97 

94 

90 

87 

84 

81 

78 

76 

73 

70 

67 

65 

110 

t. 

10.0 

2o.« 

30.0 

40.0 

50.0 

60.0 

70.0 

8o.O 

90.0 

100.0 

no.. 

120.0 

t. 

METEOROLOGICAL    TABLES. 


163 


TABLE  II.  —  RELATIVE  HUMIDITY,  PER  CENT. 


J 

Difference  between  the  dry  and  wet  thermometers  (t—f). 

I 

>a 

a 

120.0  130.0 

j 

140.0 

150.0 

160.0 

170.0 

18o.O 

190.0 

200.0 

21°.0 

220.0 

230.0 

240.0 

1 

so 

54 

51 

47 

44 

41 

38 

35 

32 

29 

26 

23 

20 

18 

80 

81 

54 

51 

48 

44 

41 

38 

35 

33 

30 

27 

24 

21 

19 

81 

82 

55 

52 

48 

45 

42 

39 

36 

33 

31 

28 

25 

22 

20 

82 

S3 

55 

52 

49 

46 

43 

40 

37 

34 

31 

29 

26 

23 

21 

83 

84 

56 

53 

49 

46 

44 

41 

38 

35 

32 

29 

27 

24 

22 

84 

1 

I 

85 

56 

53 

50 

47 

44 

41          38 

36         33 

30 

28 

25 

22 

85 

86 

57 

54 

51 

48 

45 

42         39 

36 

34 

31 

29 

26 

23 

86 

87 

57 

54 

51 

48 

45 

42 

40 

37 

34 

32 

30 

27 

24 

87 

88 

58 

55 

52 

49 

46 

43 

40 

38 

35 

32 

30 

27 

25 

88 

89 

58 

55 

52 

49 

46 

44         41 

38 

36 

33 

31 

28 

26 

89 

90 

59 

56 

53 

50 

47 

44 

41 

39 

36 

34 

32 

29 

26 

90 

91 

59 

56 

53 

50 

47 

45 

42 

39 

37 

35 

33 

30 

27 

91 

92 

59 

56 

54 

51 

48 

45 

43 

40 

37 

35 

33         30 

28 

92 

93 

60 

57 

54 

51 

48 

46 

43 

41 

38 

36 

34         31 

29 

93 

94 

60 

57 

54 

52 

49 

46 

44 

41 

39 

36 

34         31 

29 

94 

95 

60 

58 

55 

52 

49 

47 

44 

42 

39 

37 

35         32 

30 

95 

96 

61 

58 

55 

53 

50 

47 

45 

42 

40 

37 

36         33 

30 

96 

97 

61 

58 

56 

53 

50 

48 

45 

43 

40 

38 

36 

33 

31 

97 

98 

61 

59 

56 

53 

51 

48 

46 

43 

41 

38 

37 

34 

32 

98 

99 

62 

59 

56 

54 

51 

49 

46 

44 

41 

39 

37 

34 

32 

99 

100 

62 

59 

57 

54 

51 

49 

47 

44 

42 

39 

37 

35 

33 

100 

101 

62 

60 

57 

54 

52 

49 

47 

45 

42 

40 

38 

36 

33 

101 

102 

63 

60 

57 

55 

52 

50 

47 

45 

43 

40 

38 

36 

34 

10-2 

103 

63 

60 

58 

55 

53 

50 

48 

45 

43 

41 

39 

37 

34 

103 

104 

63 

61 

58 

55 

53 

51 

48 

46 

44 

41 

39 

37 

35 

104 

105 

64 

61 

58 

56 

53 

51 

49     •     46 

44 

42 

40 

38 

35 

105 

106 

64 

61 

59 

56 

54 

51 

49          47 

44 

42 

40 

38 

36 

106 

107 

64 

62 

59 

57 

54 

52 

49 

47 

45 

43 

41 

38 

36 

107 

108 

64 

62 

59 

57 

54 

52 

50 

47 

45 

43 

41 

39 

37 

108 

109 

65 

62 

60 

57 

55 

52 

50 

48 

46 

44 

41 

39 

37 

109 

110 

65 

62 

60 

57 

55 

53 

50 

48 

46 

44         42 

40 

38 

110 

i 



/. 

120.0 

130.0 

140.0 

15°.0 

160.0 

IJo.O 

180.0 

19°.0 

200.0 

210.0 

22°.0 

23°.0 

240.0 

t. 

164 


OBSERVATIONAL    METEOROLOGY. 


TABLE   II.  —  RELATIVE  HUMIDITY,  PER  CENT. 


0> 

Difference  between  the  dry  and  wet  thermometers  (t  —  f"). 

1 

1 

240.0 

250.0 

260.0 

27°.0 

280.0 

290.0 

SOo.O 

310.0 

320.0 

330.0 

340.0 

350.0  360.0 

1 

80 

18 

15 

13 

10 

8 

6 

3 

1 

80 

81 

19 

16 

14 

11 

9 

7 

4 

2 

81 

82 

20 

17 

15 

12 

10 

8 

5 

3 

1 

82 

83 

21 

18 

16 

13 

11 

9 

6 

4 

2 

83 

84 

22 

19 

17 

14 

12 

10 

8 

5 

3 

1 

84 

85 

22 

20 

17 

15 

13 

11 

9 

6 

4 

2 

85 

86 

23 

21 

18 

16 

14 

12 

10 

7 

5 

3 

1 

86 

87 

24 

22 

19 

17 

15 

13 

11 

8 

6 

4 

2 

87 

88 

25 

22 

20 

18 

16 

14 

12 

9 

7 

5 

3 

1 

88 

89 

26 

23 

21 

19 

16 

14 

12 

10 

8 

6 

4 

2 

1 

89 

90 

26 

24 

22 

20 

17 

15 

13 

11 

9 

7 

5 

3 

2 

90 

91 

27 

25 

23 

20 

18 

16 

14 

12 

10 

8 

6 

4 

3 

91 

92 

28 

26 

23 

21 

19 

17 

15 

13 

11 

9 

7 

5 

3 

92 

93 

29 

26 

24 

22 

20 

18 

16 

14 

12 

10 

8 

6 

4 

93 

94 

29 

27 

25 

23 

21 

18 

16 

14 

13 

11 

9 

7 

5 

94 

95 

30 

28 

25 

23 

21 

19 

17 

15 

13 

11 

10 

8 

(3 

95 

96 

30 

28 

26 

24 

22 

20 

18 

16 

14 

12 

10 

9 

7 

96 

97 

31 

29 

27 

25 

23 

21 

19 

17 

15 

13 

11 

10 

8 

97 

98 

32 

'  29 

27' 

25 

23 

21 

19 

18 

16 

14 

12 

10 

9 

98 

99 

32 

30 

28 

26 

24 

22 

20 

18 

16 

15 

13 

11 

10 

99 

100 

33 

31 

29 

27 

25 

23 

21 

19 

17 

15 

14 

12 

10 

100 

101 

33 

31 

29 

27 

25 

23 

21 

20 

18 

16 

14 

13 

11 

101 

102 

34 

32 

30 

28 

26 

24 

22 

20 

19 

17 

15 

13 

12 

102 

103 

34 

32 

30 

28 

26 

25 

23 

21 

19 

17 

16 

14 

12 

103 

104 

35 

33 

31 

29 

27 

25 

23 

22 

20 

18 

16 

15 

13 

104 

105 

35 

33 

31 

30 

28 

26 

24 

22 

20 

19 

17 

15 

14 

105 

106 

36 

34 

32 

30 

28 

26 

25 

23 

21 

19 

18 

16 

14 

106 

107 

36 

34 

32 

31 

29 

27 

25 

23 

22 

20 

18 

17 

15 

107 

108 

37 

35 

33 

31 

29 

27 

26 

24 

22 

21 

19 

17 

16 

108 

109 

37 

35 

33 

32 

30 

28 

26 

25 

23 

21 

20 

18 

16 

109 

110 

38 

36 

34 

32 

30 

28 

27 

25 

23 

22 

20 

19 

17 

110 

f. 

240.0 

25°.0 

26°.0 

270.0 

28°.0 

29°.0 

300.0 

310.0 

320.0 

330.0 

340.0 

350.0 

360.0 

f. 

METEOROLOGICAL   TABLES. 


165 


TABLE  II.  —  KELATIVE  HUMIDITY,  PER  CENT. 


— 
OP 

fl 

Difference  between  the  dry  and  wet  thermometers  (t  —  t'}. 

1 

L 

36°.0 

370.0 

380.0 

390.0 

400.0 

410.0 

420.0 

430.0 

440.0 

450.0 

460.0 

470.0 

4SO.O 

**' 

89 

1 

89 

90 

2 

90 

91 

3 

1     j 

91 

92 

3 

2 

1 

92 

93 

4 

3 

2 

9& 

94 

5 

4 

0 

0 

94 

95 

6 

5 

4 

1 

95 

96 

7 

5 

5 

2 

1 

96 

97 

8 

6 

6 

3 

1 

97 

98 

9 

7 

4 

2 

1 

98 

99 

10 

8 

7 

5 

3 

2 

0 

99 

100 

10 

9 

8 

8 

4 

3 

1 

100 

101 

11 

9 

9 

6 

5 

3 

2 

0 

101 

102 

12 

10 

9 

7 

6 

4 

3 

1 

102 

103 

12 

11 

9 

8 

6 

5 

4 

2 

1 

103 

104 

13 

12 

10 

8 

7 

6 

5 

3 

2 

104 

105 

14 

12 

11 

9 

8 

6 

5 

4 

2 

1 

105 

106 

14 

13 

11 

10 

8 

7 

6 

4 

3 

2 

0 

106 

107 
108 
109 

15 
16 
16 

14 
14 

15 

12 
13 
13 

11 
11 
12 

9 
10 
10 

8 
9 
9 

G 

7 
8 

5 

6 

7 

4 
5 

5 

3 
3 

4 

1 
2 
3 

1 
1 

10V 
108 
109 

110 

17 

15 

14 

13 

11 

10 

8 

7 

6 

5 

3 

2 

1 

110 

t. 

360.0 

37°.0 

380.0 

390.0 

40°.0 

410.0 

42°.0 

430.0 

440.0 

450.0 

460.0 

470.0 

480.0 

t. 

166 


OBSERVATIONAL   METEOROLOGY. 


TABLK  III.  —  REDUCTION  OF  BAROMETER  READING  TO  32°. 


Ig 

Inchrs. 

25 
1 

'24.0  |  24.5 

25.0 

ift.5 

•2<$.  0 

•_'<>..-, 

•27.0 

•27.5 

28.0 

28.5 

29.0 

29.5 

30.0 

ao.5 

«1.0 

30 

—.003  —.003  —.003 

—.003 

—  .<X« 

—.003 

—.003 

—.003 

—.003 

—.004 

—.004 

—.004 

—.004 

—.004 

—.004 

31 

.005      .005      .005 

.005 

.006 

.006 

.006 

.006 

.006      .006 

.006 

.006 

.006 

.007 

.007 

32 

.007 

.008 

.008 

.008 

.008 

.008 

.008 

.008 

.009      .009 

.009 

.009 

.009 

.009 

.009 

33 

.010      .010 

.010 

.010 

.010 

.010 

.011 

.011 

.011      .011 

.012 

.012 

.012 

.012 

.012 

34 

.012 

.012 

.012 

.012 

.013 

.013 

.013 

.013 

.014      .014 

.014 

.014 

.015 

.015 

.015 

35 

.014     ,014 

.014 

.015 

.015 

.015 

.016 

.016 

.016      .016 

.017 

.017 

.017 

.018 

.018 

36 

.016      .016 

.017 

.017 

.017 

.018 

.018 

.018 

.019      .019 

.019 

.020 

.020 

.020 

.021 

37 

.018      .019 

.019 

.019 

.020 

.020 

.021 

.021 

.021 

.022 

.022 

.022 

.023 

.023 

.024 

38 

.020      .021 

.021 

.022 

.022 

.022 

.023 

.023 

.024 

.024 

.025 

.025 

.026 

.026 

.026 

39 

.023      .023 

.024 

.024 

.024 

.025 

.025 

.026 

.026 

.027 

.027 

.028 

.028 

.029 

.029 

40 

.025      .025 

.026 

.026 

.027 

.027 

.028 

.028 

.029 

.030 

.030 

.030 

.031 

.031 

.032 

41 

.027      .027 

.028 

.029 

.029 

.030 

.030 

.031 

.031 

.032 

.033      .033 

.034 

.034 

.035 

42 

.029      .030 

.030 

.031 

.032 

.032 

.033 

.033 

.034 

.034 

.035      .036 

.036 

.037 

.038 

43 

.031      .032 

.033 

.033 

.034 

.035 

.035 

.036 

.036 

.037 

.038      .038 

.039 

.040 

.040 

44 

.033      .034 

.035 

.035 

.036 

.037 

.038 

.038 

.039 

.040 

.040,     .041 

.042 

.042 

.043 

45 

.036 

.037      .037 

.038 

.039 

.039 

.040 

.041 

.042 

.042 

.043      .044 

.045 

.045 

.046 

46 

.038 

.038 

.039 

.040      .041 

.042 

.043 

.043      .044 

.045 

.046      .046 

.047 

.048 

.049 

47 

.040 

.041 

.042 

.042      .043 

.044 

.045 

.046      .047 

.048 

.048      .049 

.050 

.051 

.052 

48 

.042 

.043 

.044 

.045 

.046 

.047 

.047 

.048 

.049 

.050 

.051 

.052 

.053 

.053 

.054 

49 

.044 

.045 

.046 

.047 

.048 

.049 

.050 

.051 

.052 

.052 

.054 

.054 

.055 

.056 

.057 

50 

.046      .047 

.048 

.049,     .050 

.051 

.052 

.053 

.054 

.055 

.056 

.057 

.058 

.059 

.060 

51 

.049      .050 

.051 

.052 

.053 

.054 

.055 

.ar>6 

.057 

.058 

.059 

.060 

.061 

.062 

.063 

52 

.051  !     .052 

.053 

.054 

.055 

.056 

.057 

.058 

.059 

.060 

.061 

.062 

.064 

.065 

.06(5 

53 

.0531     .054 

.055 

.056      .057 

.058 

.060 

.061 

.062 

.063 

.064 

.065 

.066 

.067 

.068 

54 

.055      .056 

.057 

.058      .060 

.061 

.062 

.063 

.064 

.065 

.067 

.068 

.069 

.070 

.071 

55 

.057      .058 

.060 

.061      .062 

.063 

.064 

.065 

.066 

.068 

.069 

.070 

.071 

.073 

.074 

56 

.060      .061 

.062 

.063      .064 

.065 

.067 

.068 

.069 

.070 

.072 

.073 

.074 

.075 

.077 

57 

.062      .063 

.064 

.065      .067 

.068 

.069 

.070 

.072 

.073 

.075 

.076 

.077 

.078 

.080 

58 

.064      .065 

.066      .068i     .069 

.070 

.071 

.073 

.074 

.076 

.077 

.078 

.080 

.081 

.082 

59 

.066i     .068 

.069      .070      .072 

.073 

.074 

.075 

.077 

.078 

.080 

.081 

.083 

.084 

.085 

60 

.068 

.070 

.071 

.072 

.074 

.076 

.077 

.078 

.079 

.081 

.082 

.084 

.085 

.086 

.088 

61 

.070 

.072 

.073 

.074 

.076 

.077 

.079 

.080 

.082 

.083 

.085      .086 

.088 

.089 

.091 

62 

.073 

.074 

.076 

.077 

.079 

.080 

.082 

.083 

.085 

.086 

.0881     .089 

.091 

.092 

.094 

63 

.075 

.076 

.078 

.079 

.081 

.082 

.084 

.085 

.087 

.088 

.090      .091 

.093 

.095 

.096 

64 

.077 

.078 

.080 

.081 

.083 

.085 

.086 

.088 

.090 

.091 

.0931     .094 

.096 

.097 

.099 

65 

—.079 

—.080 

—.082 

—.084 

—.086 

—.087 

—.089 

—.090 

—.092 

—.093 

—.095—  .097 

—.099 

—.100 

—.102 

METEOROLOGICAL    TABLES. 


167 


TABLE  III.  —  REDUCTION  OF  BAROMETER  READING  TO  32°.  —  Continued. 


Inches. 


J5 

24.0 

24.5 

25.0 

25.5 

26.0 

26.5 

27.0 

27.5 

28.0 

28.5 

29.0 

29.5 

30.0 

30.5 

31.0 

6fi 

—.079 

—.080 

—.082 

—.084 

—  086 

—.087 

—.089 

—.090 

—  092 

—  .093—  .095 

—.097 

—  :099 

—  100—  .102 

66 

.081 

.083 

.085 

.086 

.088 

.089 

.091      .093 

.095 

.096 

.098 

.099      .101 

.10  >      .105 

67 

.083 

.085 

.087 

.088 

.090 

.092 

.094|     .095 

.097 

.099 

.101 

.102      .104 

.106'      .108 

68 

.085 

.087 

.089 

.090 

.093 

.094 

.096      .098 

.100 

.101 

.103 

.105      .107 

.108 

.110 

69 

.088 

.089 

.091 

,.093 

.095 

.097 

.099      .100 

.102 

.104      .106 

.107      .110 

.111 

.113 

70 

.090 

.092 

.094 

.096 

.097 

.099 

.101 

.103 

.105 

.106 

.109 

.110      .112 

.114 

.116 

71 

.092 

.094 

.096 

.098 

.100 

.101 

.103]     .105 

.107 

.109 

.111 

.113      .115 

.110 

.119 

72 

.094 

.096 

.098 

.100 

.102 

.104 

.106 

.108 

.110 

.112 

.114 

.116      .118 

.120      .122 

73 

.096 

.098 

.100 

.102 

.104 

.106 

.108 

.110 

.112 

.114 

.116 

.118 

.120 

.122      .124 

74 

.098 

.100 

.103 

.105 

.107 

.109 

.111 

.113 

.115 

.117 

.119 

.121 

.123 

.125 

.127 

75 

.101 

.102 

.105 

.106 

.109 

.111 

.113 

.115 

.117 

.119 

.122 

.124 

.126 

.128 

.130 

76 

.103 

.104 

.107 

.109 

.111 

.113 

.116 

.118 

.120 

.122 

.124 

.126 

.128 

.130    .ia3 

77 

.105 

.107 

.109 

.111 

.114 

.116 

.118 

.120 

.122 

.124 

.127 

.129 

.131 

.133      .136 

78 

.107J     .109 

.112 

.113 

.116 

.118 

.120 

.122 

.125 

.127 

.129 

.131 

.134 

.136 

.138 

79 

.109 

.111 

.114 

.116 

.118 

.    .120 

.123 

.125 

.127 

.129 

.132 

.134 

.137 

.139 

.141 

80 

.111 

.113 

.116 

.118 

.121 

.123 

.125 

.127 

.130 

.132 

.133 

M37 

.139 

.141 

.144 

81 

.114 

.116 

.118 

.120 

.123 

.125 

.128 

.130 

.132 

.134 

.137 

.139 

.142 

.144 

.147 

82 

.116 

.118 

.121 

.122 

.125 

.128 

.130 

.132 

.135 

.137 

.140 

.142 

.145 

.147 

.149 

83 

.118 

.120 

.123 

.125 

.128 

.130|     .133 

.135 

.138 

.140 

.142 

.145 

.147 

.149 

.152 

84 

.120 

.122 

.125 

.127 

.130 

.132 

.135 

.138 

.140 

.142 

.145 

.147 

.150 

.152 

.155 

85 

.122 

.124 

.127 

.129 

.132 

.134 

.137 

.139 

.143 

.145 

.148 

.150 

.153 

.155 

.158 

86 

.124 

.126 

.128 

.130 

.135 

.137 

.140 

.143 

.145 

.148 

.150 

.153 

.155 

.158 

.161 

87 

.126 

.129 

.132 

.134 

.137 

.139 

.142 

.144 

.148 

.150      .153 

.155 

.158 

.161 

.163 

88 

.129 

.131 

.134 

.137 

.139 

.142 

.145 

.147 

.150 

.152 

.155 

.158 

.161 

.163 

.166 

89 

.131 

.133 

.136 

.139 

.142 

.144 

.147 

.150 

.153 

.155 

.158 

.161 

.164 

.166 

.16!) 

90 

.133 

.136 

.138 

.141 

.144 

.147 

.150 

.153 

.155 

.157 

.161 

.164 

.166 

.169 

.172 

91 

.135 

.138 

.141 

.143      .146 

.149 

.152 

.155      .158 

.160 

.163 

.166 

.169 

.172 

.175 

92 

.1371     .140 

.143 

.146      .149 

.152 

.154 

.157      .160 

.163 

.166 

.169 

.172 

.175 

.177 

93 

.139 

.142 

.145 

.148 

.151 

.154 

.157 

.160      .163 

.166 

.168 

.171 

.174 

.177 

.180 

94 

.142      .145 

.147 

.150 

.153 

.156 

.159 

.1621     .165 

.168 

.171 

.174 

.177 

.180 

.183 

95 

.144 

.147 

.150 

.153 

.156 

.159 

.162 

.165 

.168 

.171 

.174 

.177 

.180 

.183 

.186 

96 

.146 

.149 

.152 

.155 

.158 

.161 

.164 

.167      .170 

.173 

.176 

.179 

.182 

.185 

.188 

97 

.148 

.151 

.154 

.157 

.160 

.164 

.167 

.170;     .173 

.176 

.179 

.182 

.185 

.188 

.191 

98 

.150 

.153 

.156 

.160 

.163 

.166 

.169 

.172      .175 

.178 

.181 

.185 

.188 

.191 

.194 

99 

.152 

.155 

.159 

.162 

.165 

.168 

.171 

.175      .178 

.181 

.184 

.187 

.190 

.194 

.197 

100 

—.154 

—.157 

—.161 

—.164 

—.167 

—.171 

-174 

—.177 

—.180 

—.184 

—.187 

—.190 

—.193 

—.197 

—.200 

168  OBSERVATIONAL  METEOROLOGY. 

TABLE  IV. — TABLE  FOR  REDUCING  OBSERVATIONS  OF  THE  BAROMETER  TO 
SEA  LEVEL,  CORRECTION  ADDITIVE. 


.S 

^ 

t> 
H 

Temperature  of  external  air  —  degrees  Fahrenheit. 

—20o 

—10° 

0° 

10° 

20° 

300 

40° 

50o 

60o 

70° 

800 

90o 

100° 

10 

.013 

.013 

.012 

.012 

.  .012 

.012 

.011 

.011 

.011 

.011 

.010 

.010 

.010 

•20 

.020 

.025 

.025 

.024 

.023 

.023 

.023 

.022 

.022 

.021 

.021 

.020 

.020 

:',(> 

.039 

.038 

.037 

.036 

.035 

.034 

.034 

.033 

.032 

.032 

.031 

.030 

.030 

40 

.052 

.050 

.049 

.048 

.047 

.046 

.045 

.044 

.043 

.042 

.041 

.040 

.040 

60 

.065 

.063 

.061 

.060 

.059 

.058 

.056 

.055 

.054 

.053 

.052 

.051 

.050 

60 

.077 

.076 

.074 

.072 

.070 

.069 

.068 

.066 

.065 

.063 

.062 

.061 

.059 

70 

.090 

.088 

.086 

.084 

.082 

.081 

.078 

.077 

.076 

.074 

.072 

.071 

.069 

80 

.103 

.101 

.098 

.096 

.094 

.092 

».090 

.088 

.086 

.084 

.082 

.081 

.079 

90 

.116 

.113 

.111 

.108 

.105 

.104 

.101 

.099 

.097 

.095 

.093 

.091 

.089 

100 

.129 

.126 

.123 

.120 

.117 

.115 

.112 

.110 

.108 

.105 

.103 

.101 

.099 

110 

.142 

.139 

.135 

.132 

.129 

.126 

.123 

.121 

.119 

.116 

.113 

.111 

.109 

120 

.155 

.151 

.148 

.144 

.140 

.138 

.134 

.132 

.129 

.126 

.124 

.121 

.119 

130 

.168 

.164 

.160 

.156 

.152 

.149 

.146 

.143 

.140 

.137 

.134 

.131 

.129 

140 

.181 

.176 

.172 

.168 

.164 

.161 

.157 

.154 

.151 

.147 

.144 

.141 

.139 

150 

.194 

.189 

.185 

.180 

.176 

.172 

.168 

.165 

.162 

.158' 

.155 

.152 

.149 

160 

.206 

.201 

.197 

.192 

.187 

.183 

.179 

.176 

.172 

.168 

.165 

.162 

.158 

170 

.219 

.214 

.209 

.204 

.199 

.195 

.190 

.187 

.183 

.179 

.175 

.172 

.168 

180 

.232 

.227- 

.222 

.216 

.211 

.206 

.202 

.198 

.194 

.189 

.185 

.182 

.178 

190 

.245 

.239 

.234 

.228 

.222 

.218 

.213 

.209 

.204 

.200 

.196 

.192 

.188 

200 

.258 

.252 

.246 

.240 

.234 

.229 

.224 

.220 

.215 

.210 

.206 

.202 

.198 

210 

.271 

.264 

.258 

.252 

.246 

.240 

.235 

.231 

.226 

.221 

.216 

.212 

.208 

220 

.284 

.277 

.270 

.264 

.257 

.252 

.246 

.242 

.236 

.231 

.227 

.222 

.218 

230 

.296 

.289 

.283 

.276 

.269 

.263 

.257 

.253 

.247 

.242 

.237 

.232 

.228 

240 

.309 

.302 

.295 

.288 

.281 

.275 

.269 

.264 

.258 

.252 

.248 

.242 

.238 

250 

.322 

,314 

.307 

.300 

.293 

.286 

.280 

.275 

.269 

.263 

.258 

.253 

.248 

260 

.335 

.327 

.319 

.311 

.304 

.297 

.291 

.285 

.279 

.273 

.268 

.263 

.257 

270 

.348 

.339 

.331 

.323 

.316 

.309 

.302 

.296 

.290 

.284 

.278 

.273 

.267 

280 

.360 

.352 

.344 

.335 

.328 

.320 

.314 

.307 

.301 

.294 

.288 

.283 

.277 

290 

.373 

.364 

.356 

.347 

.339 

.332 

.325 

.318 

.311 

.305 

.299 

.293 

.287 

300 

.386 

.377 

.368 

.359 

.351 

.343 

.336 

.329 

.322 

.315 

.309 

.303 

.297 

310 

.399 

.389 

.380 

.371 

.363 

.354 

.347 

.340 

.333 

.326 

.319 

.313 

.307 

320 

.412 

.402 

.392 

.383 

.374 

.366 

.358 

.351 

.343 

.336 

.329 

.323 

.317 

330 

.424 

.414 

.404 

.395 

.386 

.377 

.369 

.362 

.354 

.347 

.340 

.333 

.32(> 

340 

.437 

.427 

.416 

.407 

.397 

.389 

.380 

.373 

.365 

.357 

.350 

.343 

.33(5 

350 

.450 

.439 

.429 

.419 

.409 

.400 

.392 

.384 

.376 

.368 

.360 

.353 

.346 

360 

.463 

.451 

.441 

.430 

.421 

.411 

.403 

.394 

.386 

.378 

.370 

.363 

.3511 

370 

.476 

.464 

.453 

.442 

.432 

.423 

.414 

.405 

.397 

.389 

.380 

.373 

.:)<;<; 

380 

.488 

.476 

.465 

.454 

.444 

.434 

.425 

.416 

.408 

.399 

.391 

.383 

.375 

390 

.501 

.489 

.477 

.466 

.455 

.446 

.436 

.427 

.418 

.410 

.401 

.393 

.385 

400 

.514 

.501 

.489 

.478 

.467 

.457 

.447 

.438 

.429 

.420 

.411 

.403 

.395 

410 

.527 

.513 

.501 

.490 

.479 

.468 

.458 

.449 

.440 

.430 

.421 

.413 

.405 

420 

.539 

.526 

.513 

.502 

.490 

.480 

.469 

.460 

.450 

.441 

.431 

.423 

.415 

430 

.552 

.538 

.525 

.513 

.502 

.491 

.480 

.470 

.461 

.451 

.442 

.433 

.425 

440 

.565 

.551 

.537 

.525 

.513 

..-,02 

.491 

•481 

.471 

.462 

.452 

.443 

.434 

450 

.578 

.563 

.550 

.537 

.525 

.513 

.503 

.492 

.482 

.472 

.462 

.453 

.444 

460 

.590 

.575 

.562 

.549 

.537 

.525 

.514 

.503 

.493 

.482 

.472 

.463 

.454 

470 

.603 

.588 

.:>74 

.561 

.548 

.536 

.525 

.514 

.503 

.493 

.482 

.473 

.464 

480 

.616 

.600 

.586 

.572 

.560 

.f>47 

.536 

.524 

.514 

.503 

.493 

.483 

.474 

490 

.628 

.613 

.598 

.584 

.571 

.559 

.547 

.535 

.524 

.514 

.503 

.493 

.483 

500 

.641 

.625 

.(510 

.596 

.583 

.570 

.558 

.546 

.535 

.524 

.513 

.503 

.493 

510 

.654 

.637 

.622 

.608 

.594 

.581 

.569 

.557 

.545 

.534 

.523 

.513 

.503 

520 

.666 

.650 

.634 

.620 

.606 

.593 

.580 

.568 

.556 

.545 

.533 

.523 

.513 

530 

.679 

.662 

.646 

.631 

.617 

.604 

.591 

.578 

.566 

.555 

.544 

.533 

jssa 

540 

.691 

.675 

.658 

.643 

.629 

.615 

.602 

.589 

.577 

565 

.554 

.543 

.532 

METEOROLOGICAL    TABLES. 


169 


TABLE  IV.  —  FOR  REDUCING  OBSERVATIONS  OF  THE  BAROMETER  TO  SKA 
LEVEL.  —  Continued. 


a 

ii 

D 

w 

Temperature  of  external  air  —  degrees  Fahrenheit. 

20° 

—10° 

0° 

10°. 

20° 

80° 

40° 

60° 

600 

70° 

80° 

9«o 

100° 

550 

.704 

.687 

.670 

.655 

.640 

.626 

.613 

.600 

.587 

.575 

.564 

.553 

.542 

560 

.717 

.699 

.683 

.667 

.652 

.638 

.624 

.611 

.598 

.586 

.574 

.563 

.552 

570 

.729 

.712 

.695 

.679 

.663 

.649 

.635 

.622 

.608 

.596 

.584 

.573 

.562 

580 

.742 

.724 

.707 

.690 

.675 

.660 

.646 

.632 

.619 

.606 

.595 

.583 

.571 

590 

.754 

,.737 

.719 

.702 

.686 

.672 

.657 

.643 

.629 

.617 

.605 

.593 

.581 

600 

.767 

.749 

.731 

.714 

.698 

.683 

.668 

.654 

.640 

.627 

.615 

.603 

.591 

610 

.780 

.761 

.743 

.726 

.709 

.694 

.679 

.665 

.650 

.637 

.625 

.613 

.601 

620 

.792 

.774 

.755 

.738 

.721 

.705 

.690 

.675 

.661 

.648 

.635 

.623 

.611 

630 

.805 

.786 

.767 

.749 

.732 

.717 

.701 

.686 

.671 

.658 

.645 

.633 

.620 

640 

.817 

.798 

.779 

.761 

.744 

•728 

.712 

.697 

.682 

.668 

.655 

.643 

.630 

650 

.830 

.811 

.791 

.773 

.755 

.739 

.723 

.708 

.692 

.679 

.666 

.653 

.640 

660 

.843 

.823 

.803 

.785 

.767 

.750 

.734 

.718 

.703 

.689 

.676 

.662 

.650 

670 

.855 

.835 

.815 

.797 

.778 

.761 

.745 

.729 

.713 

.699 

.686 

.672 

.660 

680 

.868 

.847 

.827 

.808 

.790 

.773 

.756 

.740 

.724 

.709 

.696 

.682 

.669 

690 

.880 

.860 

.839 

.820 

.801 

.784 

.767 

.750 

.734 

.720 

.706 

.692 

.679 

700 

.893 

.872 

.851 

.832 

.813 

.795 

.778 

.761 

.745 

.730 

.716 

.702 

.689 

710 

.905 

.884 

.863 

.844 

.824 

.806 

.789 

.772 

.755 

.740 

.726 

.712 

.698 

720 

.918 

\896 

.875 

.855 

.836 

.817 

.800 

.782 

.766 

.751 

.736 

.722 

.708 

730 

.930 

.909 

.887 

.867 

.847 

.829 

.811 

.793 

.776 

.761 

.746 

.732 

.718 

740 

.943 

.921 

.899 

.879 

.859 

.840 

.822 

.804 

.787 

.771 

.756 

.742 

.728 

750 

.955 

.933 

.911 

.891 

.870 

.851 

.833 

.815 

.797 

.782 

.767 

.752 

.738 

760 

.968 

.945 

.922 

.902 

.881 

.862 

.843 

.825 

.808 

.792 

.777 

.761 

.747 

770 

.980 

.957 

.934 

.914 

.893 

.873 

.854 

.836 

.818 

.802 

.787 

.771 

.757 

780 

.993 

.970 

.946 

.926 

.904 

.885 

.865 

.847 

.829 

.812 

.797 

.781 

.767 

790 

1.005 

.982 

.958 

.937 

.916 

.896 

.876 

.857 

.839 

.823 

.807 

.791 

.776 

800 

1.018 

.994 

.970 

.949 

.927 

.907 

.887 

.868 

.850 

.833 

.817 

.801 

.786 

810 

1.030 

1.006 

.982 

.961 

.938 

.918 

.898 

.878 

.860 

.843 

.827 

.811 

.796 

820 

1.043 

1.018 

.994 

.972 

.950 

.929 

.909 

.889 

.871 

.854 

.937 

.821 

.805 

S30 

1.055 

1.031 

1.006 

.984 

.961 

.940 

.920 

.900 

.881 

.864 

.847 

.831 

.815 

840 

1.068 

1.043 

1.018 

.995 

.973 

.951 

.931 

.911 

.892 

.874 

.857 

.841 

.825 

850 

1.080 

1.055 

1.030 

1.007 

.984 

.962 

.942 

.922 

.902 

.885 

.867 

.851 

.835 

S60 

1.093 

1.067 

1.041 

1.019 

.995 

.974 

.952 

.932 

.913 

.895 

.877 

.860 

.844 

870 

1.105 

1.079 

1.053 

1.030 

1.007 

.985 

.963 

.943 

.923 

.905 

.887 

.870 

.854 

880 

.118 

1.092 

1.065 

1.042 

1.018 

.996 

.974 

.954 

.934 

.915, 

.897 

.880 

.864 

890 

1.130 

1.104 

1.077 

1.053 

1.030 

1.007 

.985 

.964 

.944 

.926 

.907 

.890 

.873 

900 

.143 

1.116 

1.089 

1.065 

1.041 

1.018 

.996 

.975 

.955 

.936 

.917 

.900 

.883 

910 

1.155 

1.128 

.101 

1.077 

1.052 

1.029 

1.007 

.986 

.965 

.946 

.927 

.910 

.893 

920 

1.168 

1.140 

.113 

1.088 

1.064 

1.040 

1.018 

.996 

.976 

.956 

.937 

.920 

.902 

930 

1.180 

1.152 

1.125 

1.100 

1.075 

1.051 

1.029 

1.007 

.986 

.967 

.947 

.929 

.912 

940 

1.193 

1.164 

1.137 

1.111 

1.086 

1.062 

1.040 

1.017 

.997 

.977 

.957 

.939 

.921 

950 

1.205 

.177 

1.149 

1.123 

1.098 

1.074 

1.051 

1.028 

1.007 

.987 

.967 

.949 

.931 

960 

1.217 

.189 

1.160 

1.135 

1.109 

1.085 

1.061 

1.039 

1.017 

.997 

.977 

.959 

.941 

970 
980 

1.230 
1.242 

.201 
.213 

1.172 
1.184 

1.146 

1.158 

1.120 
1.131 

1.096 
1.107 

1.072 
1.083 

1.049 
1.060 

1.028 
1.038 

1.007 
1.018 

.987 
.997 

.969 
.978 

.950 
.960 

990 

1.255 

.225 

1.196 

1.169 

1.143 

1.118 

1.094 

1.070 

1.049 

1.028 

1.007 

.988 

.969 

1000 
1010 

1.267 
1.279 

1.237 
1.249 

1.208 
1.220 

1.181 
1.192 

1.154 
1.165 

1.129 
1.140 

1.105 
1.116 

1.081 
1.092 

1.059 
1.069 

1.038 
1.048 

1.017 
1.027 

.998 
1.008 

.979 
.989 

1020 
1030 
1040 

1.292 
1.304 
1.317 

1.261 
1.273 
1.285 

1.232 
1.243 
1.255 

1.204 
1.215 
1.227 

1.177 
1.188 
1.199 

1.151 
1.162 
1.173 

1.127 
1.137 
1.148 

1.102 
1.113 
1.123 

1.080 
1.090 
1.101 

1.058 
1.069 
1.079 

1.037 
1.047 
1.057 

1.018 
1.027 
1.037 

.998 
1.008 
1.017 

1050 
1060 
1070 
1080 
1090 

1.329 
1.341 
1.354 
1.366 
1.379 

1.298 
1.310 
1.322 
1.334 
1.346 

1.267 
1.279 
1.291 
1.302 
1.314 

1.238 
1.250 
1.261 
1.273 
1.284 

1.211 
1  22^ 
l!233 
1.244 
1.256 

1.184 
1.195 
1.206 
1.217 

1.228 

1.159 
1.170 
1.181 
1.191 
1.202 

1.134 
1.145 
1.155 
1.166 
1.176 

1.111 
1.121 
1.132 
1.142 
1.153 

1.089 
1.099 
1.109 
1.120 
1.130 

1.067 
1.077 
1.087 
1.097 
1.107 

1.047 
1.057 
1.067 
1.076 
1.086 

1.027 
1.037 
1.046 
1.056 
1.065 

170 


OBSERVATIONAL   METEOROLOGY. 


TABLE  IV.  —  FOR  REDUCING  OBSERVATIONS  OF  THE  BAROMETER  TO  SEA 
LEVEL.  —  Continued. 


a 

fi 

Temperature  of  external  air  —  degrees  Fahrenheit. 

—20° 

—10° 

0° 

10° 

20° 

30° 

40° 

50° 

60° 

70° 

800 

90° 

100° 

1100 

1.391 

1.358 

1.326 

1.296 

1.267 

1.239 

1.213 

1.187 

1.163 

1.140 

1.117 

1.096 

1.075 

1110 

1.403 

1.370 

1.338 

1.307 

1.278 

1.250 

1.224 

1.198 

1.173 

1.150 

1.127 

1.106 

1.085 

1120 

1.416 

1.382 

1.350 

1.319 

1.289 

1.261 

1.235 

1.208 

1.184 

1.160 

.137 

1.115 

1.094 

1130 

1.428 

1.394 

1.361 

1.330 

1.301 

1.272 

1.245 

1.219 

1.194 

1.170 

.147 

1.125 

1.104 

1140 

1.440 

1.406 

1.373 

1.342 

1.312 

1.283 

1.256 

1.229 

1.204 

1.180 

.157 

1.135 

1.113 

1160 

1.453 

1.418 

1.385 

1.353 

1.323 

1.294 

1.267 

1.240 

1.215 

1.191 

.167 

1.145 

1.123 

1160 

1.465 

1.430 

1.397 

1.365 

1.334 

1.305 

1.278 

1.251 

1.225 

1.201 

.177 

1.154 

1.133 

1170 

1.477 

1.442 

1.409 

1.376 

1.345 

1.315 

1.289 

1.261 

1.235 

1.211 

.187 

1.164 

1.14'J 

1180 

1.489 

1.454 

1.420 

1.388 

1.357 

1.327 

1.299 

1.272 

1.245 

1.221 

1.197 

1.174 

1.152 

1190 

1.502 

1.466 

1.432 

1.399 

1.368 

1.338 

1.310 

1.282 

1.256 

1.231 

1.207     1.183 

1.161 

1200 

1.514 

1.478 

1.444 

1.411 

1.379 

1.349 

1.321 

1.293 

1.266 

1.241 

1.217 

1.193 

1.171 

1210 

1.526 

1.490 

1.456 

1.422 

1.390 

1.360 

1.332 

1.303 

1.276 

1.251 

1.227 

1.203 

1.180 

1220 

1.539 

1.502 

1.467 

1.434 

1.401 

1.371 

1.342 

1.314 

1.288 

1.261 

1.237 

1.212 

1.190 

1230 

1.551 

1.514 

1.479 

1.445 

1.413 

1.382 

1.353 

1.324 

1.297 

1.271 

1.247 

1.222 

1.199 

1240 

1.563 

1.526 

1.491 

1.457 

1.424 

1.393 

1.364 

1.335 

1.307 

1.281 

1.257 

1.232 

1.209 

1250 

1.576 

1.538 

1.502 

1.468 

1.435 

1.404 

1.374 

1.345 

1.317 

1.291 

1.266 

1.242 

1.218 

1260 

1.588 

1.550 

1.514 

1.479 

1.446 

1.415 

1.385 

1.356 

1.328 

1.302 

1.276 

1.251 

1.228 

1270 

1.600 

1.562 

1.526 

1.491 

1.457 

1.426 

1.396 

1.366 

1.338 

1.312 

1.286 

1.261 

1.237 

1280 

1.612 

1.574 

1.538 

1.502 

1.469 

1.437 

1.407 

1.377 

1.348 

1.322 

1.296 

1.271 

1.247 

1290 

1.625 

1.586 

1.549 

1.514 

1.480 

1.448 

1.417 

1.387 

1.&59 

1.332 

1.306    1.280 

1.256 

1300 

1.637 

1.598 

1.561 

1.525 

1.491 

1.459 

1.428 

1.398 

1.369 

1.342 

1.316    1.290 

1.266 

1310 

1.649 

1.610 

1.573 

1.536 

1.502 

1.470 

1.439 

1.408 

1.379 

1.352 

1.326 

1.300 

1.275 

1320 

1.661 

1.622 

1.584 

1.548 

1.513 

1.481 

1.449 

1.419 

1.390 

1.362 

1.336 

1.309 

1.285 

1330 

1.674 

1.634 

1.596 

1.559 

1.525 

1.492 

1.460 

1.429 

1.400 

1.372 

1.346 

1.319 

1.294 

1340 

1.686 

1.646 

1.608 

1.571 

1.536 

1.503 

1.471 

1.440 

1.410 

1.382 

1.356 

1.329 

1.304 

1350 

1.698 

1.658 

1.620 

1.582 

1.547 

1.514 

1.482 

1.450 

1.420 

1.393 

1.366 

1.339 

1.313 

1360 

1.710 

1.669 

1.631 

1.593 

1.558 

1.524 

1.492 

1.461 

1.431 

1.403 

1.375 

1.348 

1.323 

1370 

1.722 

1.681 

1.643 

1.605 

1.569 

1.535 

1.503 

1.471 

1.441 

1.413 

1.385 

1.358 

1.332 

1380 

1.735 

1.693 

1.655 

1.616 

1.581 

1.546 

1.514 

1.482 

1.451 

1.423 

1.395 

1.368 

1.342 

1390 

1.747 

1.705 

1.666 

1.628 

1.592 

1.557 

1.524 

1.492 

1.462 

1.433 

1.405 

1.377 

1.351 

1400 

1.759 

1.717 

1.678 

1.639 

1.603 

1.568 

1.535 

1.503 

1.472 

1.443 

1.415 

1.387 

1.361 

1410 

1.771 

1.729 

1.690 

1.650 

1.614 

1.579 

1.546 

1.513 

1.482 

1.453 

1.425 

1.397 

1.370 

1420 

1.783 

1.741 

1.701 

1.662 

1.625 

1.590 

1.556 

1.524  k  1.492 

1.463 

1.435 

1.406 

1.380 

1430 

1.796 

1.753 

1.713 

1.673 

.636 

1.601 

1.567 

1.534     1.503 

1.473 

1.444 

1.416 

1.389 

1440 

1.808 

1.765 

1.724 

1.685 

.647 

1.612 

1.577 

1.545 

1.513 

1.483 

1.454 

1.426 

1.399 

1450 

1.820 

1.777 

1.736 

1.696 

i.ar>8 

1.623 

1.588 

1.555 

1.523 

1.493 

1.464 

1.436 

1.408 

1460 

1.832 

1.788 

1.748 

1.707 

.670 

1.633 

1.599 

1.565 

1.533 

1.503 

1.474 

1.445 

1.418 

1470 

1.844 

1.800 

1.759 

1.719 

.681 

1.644 

1.609 

1.576 

1.543 

1.513 

1.484 

1.455 

1.427 

1480 

1.857 

1.812 

1.771 

1.730 

.692 

1.655 

1.620 

1.586 

1.554 

1.523 

1.493 

1.465 

1.437 

1490 

1.869 

1.824 

1.782 

1.742 

1.703 

1.666 

1.630 

1.597 

1.564 

1.533 

1.503 

1.474 

1.446 

1500 

1.881 

1.836 

1.794 

1.753 

1.714 

1.677 

1.641 

1.607 

1.574 

1.543 

1.513 

1.484 

1.456 

APPENDIX    A. 


SUGGESTIONS   TO   TEACHERS. 

IT  is  the  object  of  this  book  to  lead  the  student  to  the  independ- 
ent discovery  of  the  most  important  facts  in  our  ordinary  weather 
conditions,  and  of  the  interrelations  of  the  different  weather  ele- 
ments. This  practical  study  having  taught  something  as  to  the 
real  nature  of  atmospheric  phenomena  by  actual  observation,  rapid 
and  substantial  progress  may  be  made  in  the  knowledge  of  the 
distribution  and  of  the  explanation  of  similar  phenomena  in  other 
parts  of  the  world,  as  derived  through  a  study  of  the  text-books. 
By  means  of  this  combination  of  the  two  kinds  of  study,  the  induc- 
tive and  the  didactic,  the  advantages  of  both  may  be  preserved,  and 
the  slow  progress  of  the  first  method  and  the  unsound  progress  of 
the  second  may  be  avoided.  This  book  is  not  a  text-book,  and  it 
therefore  does  not  attempt  to  give  explanations  of  various  phe- 
nomena discovered  by  the  class.  Explanations  will,  of  course,  be 
called  for  by  the  scholars,  in  increasing  number  as  the  work  pro- 
gresses, and  the  larger  relations  of  the  study  become  apparent.  It 
is  best,  if  possible,  to  leave  the  more  complicated  matters  (such 
as  the  cause  of  the  deflection  of  the  wind  from  the  gradient,  of 
cyclones  and  anticyclones,  etc.)  until  the  subjects  can  be  taken  up 
in  detail  and  fully  explained,  for  instance  in  the  later  years  of  the 
high  school  course.  It  is  not  advisable  to  raise  such  complicated 
questions  in  the  grammar  school  work  if  they  can  be  avoided. 
The  teacher  who  has  a  fairly  good  knowledge  of  one  comprehensive 
modern  text-book  of  meteorology,  such  as  Davis's  Elementary 
Meteorology,  will  find  himself  sufficiently  well  equipped  to  answer 
the  questions  put  by  the  class. 

The  value  of  the  work  outlined  in  this  little  book  can  be  much 
increased  if  the  larger  applications  of  the  lessons  here  learned  are 
strongly  emphasized.  Suggestions  along  this  line  have  been  made 

171 


172  APPENDIX   A. 

in  fine  print  throughout  the  text,  but  the  examples  given  may  be 
further  extended  to  the  great  advantage  of  the  student.  Careful 
attention  ought  to  be  given  to  the  formulating  and  writing  out  of 
the  generalizations  reached  by  the  class,  for  in  these  written  sum- 
maries the  results  are  preserved  in  compact  form. 

CHAPTER   I. 

The  work  outlined  in  this  chapter  is  adapted  to  the  lower  grades 
in  the  grammar  school.  It  is  assumed  that  the  pupils  have  already 
had  some  preliminary  training  in  the  simplest  non-instrumental 
weather  observations,  such  as  can  readily  be  made  during  the  pri- 
mary school  years.  For  the  convenience  of  teachers  who  may  desire 
it,  a  brief  outline  of  work  suited  to  the  primary  school  grades  is  here 
given.  It  is  desirable  that  even  older  scholars  be  given  some  such 
training  as  this  before  they  take  up  the  exercises  of  Chapter  II. 

The  central  idea  in  this  elementary  work  is  to  train  the  children 
in  intelligent  weather  observation,  so  that  they  may  come  to  appre- 
ciate what  our  typical  weather  changes  are ;  that  they  may  recognize 
the  types  as  they  recur,  and  may  see  how  each  example  differs 
from,  or  accords  with,  those  that  have  preceded  it.  We  are  all  so 
directly  affected  by  the  weather  conditions  prevailing  at  any  time 
that  even  the  youngest  children  are  forced,  unconsciously  to  be 
sure,  to  take  some  notice  of  these  changes.  The  work  of  the 
teacher  is,  therefore,  simply  to  direct  attention  to  what  is  already 
seen. 

When  the  children  come  to  school  on  some  snowy  winter  day, 
with  a  northeast  wind,  chilling  and  damp,  attention  may  be  called 
to  the  need  of  overshoes  and  overcoats,  to  the  piling  up  of  the  snow 
in  deep  drifts  at  certain  places  near  the  school  or  in  the  town,  while 
in  other  places  the  ground  is  left  bare ;  to  the  ease  with  which  snow- 
balls may  be  made,  and  to  other  facts  which  will  very  readily  sug- 
gest themselves.  A  day  or  two  after  such  a  storm,  when  the  sun  is 
shining  bright  in  a  cloudless  sky.  when  there  is  no  wind  and  the 
air  is  dry,  cold,  and  crisp,  the  contrasts  between  these  two  weather 
types  should  be  brought  out.  Instead  of  snow  we  now  have  sun- 
shine ;  instead  of  a  damp,  chilling  northeaster  we  now  have  a  calm 


APPENDIX   A.  173 

and  the  air  is  dry ;  snowballs  cannot  easily  be  made  in  the  early 
mornjng  because  the  snow  is  frozen  hard  and  is  too  dry,  but  towards 
noon,  if  the  temperature  rise  high  enough,  there  may  be  thawing  on 
the  tops  or  sides  of  the  snowdrifts,  and  there  the  snow  becomes  soft 
enough  for  snowballing.  Another  weather  type,  often  noted  during 
our  winter  in  the  central  and  eastern  United  States,  and  strongly 
contrasted  with  both  of  the  preceding  conditions,  is  that  which 
brings  us  a  warm,  damp,  southerly  wind,  frequently  accompanied 
by  heavy  rains.  As  these  damp  winds  blow  over  snow-covered  sur- 
faces they  become  foggy  and  the  ground  is  said  to  "  smoke " ;  the 
heavy  rain  rapidly  melts  the  snow  ;  slush  and  mud  make  bad  walk- 
ing ;  rivers  and  brooks  rise  rapidly,  perhaps  overflowing  their  banks  ; 
low-lying  places  become  filled  with  standing  water.  These  and 
other  features  should  all  be  brought  out  by  the  teacher,  not  by  tell- 
ing the  class  of  them  directly,  but  by  judicious  questioning,  and 
they  should  be  contrasted  with  the  conditions  which  may  immedi- 
ately follow,  when  the  storm  has  cleared  off,  and  when  the  low 
temperatures  brought  by  a  cold  wave,  with  its  dry  northwest 
wind,  have  resulted  in  freezing  lakes,  rivers,  and  brooks,  and  when 
skating  and  sliding  may  be  indulged  in.  Early  summer  weather 
conditions,  with  their  characteristic  warm  spells,  cumulus  clouds, 
thunderstorms,  and  (near  the  coast)  sea  breezes,  furnish  another  long 
list  of  typical  changes  that  should  be  just  as  carefully  noted  and 
described  as  the  more  striking  winter  characteristics.  Autumn 
types  add  further  to  the  list,  which  might  be  extended  almost 
indefinitely. 

One  whole  year  of  the  grammar  school  course  may  well  be  given 
to  the  observations  suggested  in  Chapter  I,  provided  that  there  is  no 
need  of  hastening  on  to  the  more  advanced  work.  The  advantage  of 
extending  the  course  over  a  whole  school  year  is  great,  because  such 
extension  gives  opportunity  for  becoming  familiar  with  late  summer, 
autumn,  winter,  spring,  and  early  summer  weather  types,  and  this  is 
far  better  than  attempting  to  crowd  all  the  work  into  one  short  season 
The  interest  of  a  class  can  easily  be  kept  up  throughout  a  school 
year  by  means  of  a  progressive  system  of  observations.  It  is  best 
to  vary  the  observations  from  time  to  time,  and  to  arrange  them  so 
that,  beginning  with  the  more  simple,  they  shall  gradually  become 


174  APPENDIX    A. 

more  complete  and  more  advanced  as  the  year  goes  on.  Thus,  start- 
ing with  temperature  observations  alone,  these  may  be  continued 
for  one  or  two  weeks  before  they  are  supplemented  by  records  of 
wind  direction  and  velocity.  After  some  practice  in  the  observation 
of  these  two  weather  elements  (say  during  one  month),  data  as  to 
the  state  of  the  sky  may  be  added.  Cloud  observations  themselves 
may  well  be  graded  during  successive  weeks,  so  that,  beginning 
with  the  simplest  notes  concerning  amounts  of  cloudiness,  the  pupils 
shall  gradually  advance  to  the  point  of  observing,  and  perhaps 
even  of  sketching,  the  common  cloud  forms  and  their  changes. 
Thus  an  important  step  will  have  been  taken  towards  appreciating 
the  need  of  a  standard  cloud  classification,  which  may  be  given 
later. 

The  addition  of  records  of  precipitation  completes  the  list  of 
simple  non-instrumental  weather  observations,  and  these  records,  as 
well  as  the  cloud  records,  can  easily  be  graded,  so  that,  during  suc- 
cessive weeks,  every  week's  work  shall  be  different  from  that  of 
every  other  week.  In  this  progression  from  the  simpler  to  the 
more  complicated  observations  lies  the  secret  of  making  the  work 
attractive.  Nothing  will  sooner  check  interest  in  the  study  than 
the  necessity  of  making  exactly  the  same  observations  day  after  day 
and  week  after  week  throughout  the  year.  A  graded  course  of  non- 
instrumental  observation,  such  as  is  suggested,  gives  a  very  practical 
general  knowledge  of  our  common  weather  types  and  changes,  and 
of  the  relations  of  one  weather  element  to  another.  The  questions 
asked  under  the  different  headings  in  this  chapter  are  designed  to 
awaken  the  interest  of  the  scholars,  and  to  call  their  attention 
to  the  more  important  points  of  diurnal,  cyclonic,  and  seasonal 
changes  in  weather  elements.  The  teacher  will  readily  think  of 
other  questions  which  may  be  suggested  for  the  consideration  of  the 
class. 

Although  the  non-instrumental  records  are  of  little  value  for 
future  reference,  as  compared  with  the  instrumental  observations, 
they  should  nevertheless  be  systematically  preserved  by  the  class  in 
their  record  books.  After  discussion  of  the  daily  observations  made 
by  the  different  scholars,  or  by  one  of  their  number,  the  records 
may  be  written  upon  one  of  the  blackboards  reserved  for  this  pur- 


APPENDIX    A.  175 

pose.  At  the  close  of  the  day,  or  the  next  morning,  the  blackboard 
notes  should  be  entered  in  a  record  book  kept  in  the  schoolroom. 
The  teacher  may  guide  in  the  discussion  of  the  observations ;  may 
suggest  points  overlooked  by  the  scholars ;  may  draw  comparisons 
between  the  weather  conditions  of  other  weeks  and  of  other  days. 
This  talking  over  of  the  observations  is  most  important,  as  it  never 
fails  to  bring  out  much  of  interest. 


CHAPTER   TI. 

This  work  may  usually  be  begun  in  the  early  years  of  the  gram- 
mar school  course,  as  soon  as  the  non-instrumental  observations  have 
been  satisfactorily  completed.  The  scheme  of  progressive  observa- 
tions already  suggested  may  be  followed  to  advantage  in  the  instru- 
mental work  as  well  as  in  the  non-instrumental.  It  is  often  a  good 
plan  to  have  a  different  scholar  assigned  to  the  task  of  taking  the 
observations  every  day,  or  it  may  be  more  advisable  to  divide  the 
work,  making  one  responsible  for  the  temperature  observations, 
another  for  the  precipitation,  etc.  It  is  well  to  have  the  daily 
instrumental  weather  records  written  upon  the  blackboard  in  the 
schoolroom,  as  already  suggested  in  the  case  of  the  non-instrumental 
observations.  At  the  end  of  each  day  the  blackboard  data  should 
be  entered  in  a  permanent  record  book  by  some  one  of  the  scholars, 
and  some  ingenuity  can  be  exercised  in  devising  the  best  scheme 
for  keeping  this  record.  The  record  book  should  be  carefully  pre- 
served in  the  schoolroom,  where  it  may  be  referred  to  by  the 
scholars  of  future  years  when  any  unusually  severe  storm,  or  a  spell 
of  excessively  hot  or  dry  weather,  or  a  remarkable  cold  wave  occurs, 
in  order  that  comparison  with  past  occurrences  of  a  similar  kind 
may  be  made.  It  is  well  to  have  the  record  book  of  large  size,  and 
to  have  each  day's  record  entered  across  two  full  pages.  On  the 
left-hand  page  the  temperature,  pressure,  rainfall,  wind  direction  and 
velocity,  etc.,  may  be  entered,  each  observation  in  its  proper  column, 
the  number  of  columns  being  increased  according  to  the  increasing 
number  of  observations.  The  right-hand  page  may  be  left  for 
"  Remarks."  These  "  Remarks  "  should  include  notes  of  any  meteor- 
ological phenomena  which  did  not  find  a  place  in  the  columns 


176  APPENDIX    A. 

reserved  for  the  regular  observations,  e.g.,  occurrence  of  hail,  or 
frozen  rain  ;  damage  by  lightning,  winds,  or  floods ;  freezing  up  of 
rivers  or  brooks;  interruption  of  railroad  or  street-car  traffic  by 
snow,  etc.,  and,  in  general,  explanatory  comments  on  the  weather 
conditions.  Instructive  lessons  may  be  taught  as  to  the  relation  of 
the  local  weather  conditions  which  prevail  in  the  vicinity  of  the 
school,  and  those  of  other  portions  of  the  country,  by  comments  on 
newspaper  despatches  concerning  gales  along  the  coast  or  on  the 
lakes,  and  resulting  damage  to  shipping ;  of  snow  blockades  and 
stalled  trains  ;  of  sqvere  thunderstorms  and  tornadoes ;  of  hot  waves 
and  sunstrokes,  or  of  cold  waves  and  the  destruction  of  crops  or 
fruits  by  the  frost.  The  scholars  should  be  encouraged  to  bring 
into  the  class  any  comments  on  such  phenomena  as  may  be  of 
interest  in  the  work.  Such  of  these  newspaper  clippings  as  are 
of  the  most  value  may  be  pasted  in  the  space  reserved  for  the 
"  Remarks,"  where  they  may  be  referred  to  by  succeeding  classes ; 
and  in  this  space  also  may  be  pasted  at  the  end  of  each  week  the 
barograph  and  thermograph  sheets,  if  these  instruments  are  in  use 
at  the  school. 

CHAPTER    ITT. 

These  observations  may  usually  be  profitably  undertaken  in  the 
later  grammar  and  in  the  high  school  years.  The  instruments 
described,  while  all  desirable,  are  by  no  means  all  necessary,  and  no 
teacher  should  postpone  the  establishment  of  a  course  in  observa- 
tional meteorology  for  the  reason  that  a  complete  set  of  first-class 
instruments  cannot  be  secured  at  the  start. 

If  the  school  is  provided  with  a  psychrometer,  there  will  be  no 
need  of  the  ordinary  thermometer,  because  the  psychrometer  gives 
the  true  air  temperature.  It  is  well,  however,  to  have  both  station- 
ary wet  and  dry  bulb  thermometers,  in  the  shelter,  for  ordinary 
school  use,  and  also  a  sling  psychrometer  for  use  in  the  meteorological 
field  work  which  forms  an  important  part  of  the  more  advanced 
instrumental  work  in  meteorology.  The  sling  psychrometer  may, 
of  course,  be  used  simply  as  an  ordinary  sling  thermometer. 

The  simple  form  of  mercurial  barometer,  without  vernier  and  with- 


APPENDIX   A.  177 

out  attached  thermometer,  described  in  Chapter  II,  will  be  found 
the  best  barometer  for  general  school  use.  The  standard  barometer, 
described  in  this  chapter,  is  too  expensive  and  too  complicated  to 
come  into  extended  use  in  our  schools.  Full  instructions  concern- 
ing the  care,  the  reading,  and  the  corrections  of  the  standard  mercurial 
barometer  are  published  by  the  Weather  Bureau,  and  to  these  in- 
structions teachers  who  have  such  an  instrument  are  referred.  (See 
Appendix  B.) 

The  form  of  table  given  at  the  end  of  this  chapter  is  intended 
merely  as  a  suggestion,  and  not  as  a  rigid  scheme  to  be  adopted  in 
every  school.  In  using  the  instruments  here  described,  practice 
with  the  maximum  and  minimum  thermometers  (in  addition  to  the 
simpler  work  of  Chapter  II)  may  be  given  before  any  attempt  is  made 
to  have  the  class  use  the  psychrometer.  And  in  using  the  psychrom- 
eter  one  week  may  well  be  given  to  the  determination  of  the  dew- 
point  alone,  before  the  wet  and  dry  bulb  readings  are  employed  to 
determine  the  relative  humidity.  Absolute  humidity,  which  is 
not  referred  to  in  this  chapter,  may,  if  the  teacher  deem  it  ad- 
visable, be  added  as  another  weather  element  for  study.  A  re- 
finement in  the  notes  on  the  state  of  the  sky  is  suggested,  viz., 
that  cloudiness  should  be  recorded  in  tenths  of  the  sky  covered 
by  clouds.  This  is  an  advance  over  the  earlier,  less  accurate 
cloud  observations,  and  is  in  line  with  such  a  progressive  scheme 
as  has  been  recommended.  This  book  is  not  intended  to 
present  a  rigid  scheme  of  observational  work  in  meteorology, 
alike  for  all  schools,  but  rather  to  make  suggestions  for  the  guid- 
ance of  teachers  in  laying  out  such  a  course  as  may  fit  their  own 
cases. 

Under  the  heading  Summary  of  Observations  only  a  few  of  the 
most  important  climatic  elements  have  been  noted.  The  list  may 
easily  be  extended  by  the  addition  of  such  data  as  the  following : 
For  temperature,  mean  diurnal  range  ;  mean  diurnal  variability  (the 
mean  of  the  differences  between  the  successive  daily  means).  For 
humidity,  monthly  mean  absolute  humidity.  For  precipitation, 
the  maximum  daily  precipitation ;  the  number  of  rainy  and  snowy 
days  in  every  month,  the  number  of  clear,  fair,  and  cloudy  days 
in  every  month;  the  mean  frequency  of  rainfall  in  every  month 


178  APPENDIX    A. 

(number  of  rainy  days  divided  by  the  total  number  of  days)  ;  the 
number  of  days  with  thunderstorms,  etc. 

It  is  important  that  the  monthly  summaries  should  be  discussed 
in  the  class,  and  that  the  scholars  should  give  verbal  statements  as 
to  the  numerical  results  which  they  have  obtained.  In  this  way  the 
work  will  have  a  living  interest,  which  the  mere  compilation  of  sum- 
maries does  not  possess. 

CHAPTER   IV. 

The  first  thing  for  any  teacher  to  do  who  intends  to  establish  a 
course  in  meteorology  is  to  secure  a  supply  of  daily  weather  maps. 
Arrangements  should  be  made  to  have  them  mailed  regularly  from 
the  nearest  map-publishing  station  of  the  Bureau.  It  is  important 
that  the  Saturday  morning  map,  which  is  usually  not  sent  to  schools, 
should  be  included  in  the  set,  as  the  break  of  two  days  (Saturday 
and  Sunday)  in  every  week  seriously  interferes  with  the  value  of 
the  work  that  may  be  done  on  consecutive  maps.  The  maps  should 
be  securely  fastened  up  in  the  schoolroom  or  in  the  hall.  It  is 
advisable  to  keep  at  least  two  maps  thus  on  view  all  the  time,  in 
order  that  the  scholars  may  be  able  to  study  the  changes  from  day 
to  day  by  comparing  the  last  two  or  three  maps  with  one  another. 
As  soon  as  they  are  removed  from  the  wall,  the  maps  should  be 
carefully  filed  away  for  future  reference.  They  may  be  conven- 
iently kept  in  stiff  brown  paper  folders,  each  month's  maps  being 
enclosed  in  a  separate  folder,  with  the  name  of  the  month  and  the 
year  written  on  the  outside.  It  is  a  good  plan  to  keep  with  the  file 
of  maps  any  newspaper  clippings  referring  to  notable  meteorological 
phenomena  associated  with  the  conditions  shown  on  the  maps. 
Thus,  newspaper  accounts  of  the  damage  done  by  a  hurricane  along 
the  Atlantic  Coast ;  of  the  blockades  caused  by  a  heavy  snowstorm 
in  the  Northwest ;  of  tornadoes  in  the  Mississippi  Valley ;  of  hot 
waves  and  sunstrokes  in  our  larger  cities,  will  serve  to  enliven  the 
study  of  the  maps,  and  will  also  help,  in  later  references  to  them,  to 
recall  interesting  points  that  might  otherwise  escape  the  memory. 
It  is  true  that  newspapers  are  prone  to  exaggerate,  and  that  they 
are  lamentably  inaccurate  in  their  use  of  meteorological  terms ;  but 


APPENDIX    A.  179 

nevertheless  they  may  often  be  profitably  used  in  such  general 
studies  as  these.  Besides  the  complete  weather  map,  the  school  will 
need  a  supply  of  blank  weather  maps,  as  used  by  the  Weather 
Bureau.  These  may  usually  be  secured  from  the  nearest  map- 
publishing  station  at  cost  price.  In  the  case  of  the  preparation  of 
illustrations  for  permanent  class  use,  as  suggested  later,  it  is  advis- 
able to  employ  the  blank  maps  used  as  the  base  of  the  Washington 
daily  weather  maps,  and  to  be  obtained,  at  cost  price,  on  application 
to  the  Chief  of  the  Weather  Bureau,  Washington,  D.  C.  These 
are  larger  in  size  (16£  by  23f  inches)  than  the  station  maps,  and 
the  paper  is  of  a  better  quality.  Colored  illustrations  with  these 
Washington  blank  maps  as  the  base  furnish  an  economical,  simple, 
and  effective  means  of  teaching  elementary  meteorology. 

CHAPTER   V. 

In  this  chapter  a  series  of  six  consecutive  weather  maps  is  taken 
as  the  basis  of  the  work.  The  study  of  the  weather  elements  on 
such  a  series  of  maps  gives  a  far  clearer  understanding  of  the  dis- 
tribution of  these  elements  and  of  their  relation  one  with  another, 
than  if  a  far  larger  number  of  single  maps  are  studied  which  do 
not  follow  one  another  in  regular  sequence.  Teachers  should  add 
to  the  map-drawing  exercises  by  giving  their  classes  the  data  from 
other  sets  of  maps  selected  from  the  school  files.  Summer  maps  as 
well  as  winter  ones  should  be  used,  in  order  that  too  much  emphasis 
may  not  be  laid  on  winter  conditions.  The  search  for  the  various 
cities  in  which  Weather  Bureau  stations  are  established,  involved 
in  the  work  of  entering  the  data  on  the  blank  maps,  furnishes  excel- 
lent practice  in  geography.  This  exercise  may  be  varied,  and  prac- 
tice in  the  location  of  the  different  States  may  be  given,  if  the 
teacher  reads  out  to  the  class  the  temperatures  in  different  parts  of 
the  various  States,  as,  e.g.,  central  Arizona,  34° ;  southwestern  Ten- 
nessee, 30°  ;  northern  Nevada,  38°,  etc.  As  the  State  names  are  not 
given  on  the  Weather  Bureau  maps,  such  a  method  as  this  will  give 
a  very  desirable  familiarity  with  the  relative  positions  of  the  States 
of  the  Union.  If  the  school  possesses  a  blackboard  outline  map  of 
the  United  States,  it  may  be  a  good  plan,  if  the  class  is  small,  to 


180  APPENDIX    A. 

have  one  of  the  pupils  enter  the  temperatures  and  draw  the  isotherms 
on  the  board  before  the  class,  and  to  let  the  others  correct  him  if 
they  see  that  he  is  going  wrong.  As  to  irregularities  in  the  iso- 
therms, which  may  cause  trouble,  the  officials  of  the  Weather 
Bureau  vary  somewhat  among  themselves  in  dealing  with  such 
cases,  and  no  definite  rules  can  be  laid  down  to  fit  all  occasions.  It 
is  best  to  select  maps  with  few  irregularities  at  first,  and  in  time 
experience  will  show  how  the  exceptional  cases  may  be  treated. 

By  the  scheme  of  coloring  isothermal  maps,  suggested  in  this 
chapter,  a  valuable  series  of  permanent  illustrations  of  noteworthy 
weather  types  for  class  use  can  readily  be  prepared  at  very  slight  cost. 
For  the  purposes  of  these  colored  illustrations  it  is  best  to  use  the 
large  scale  Washington  blank  weather  maps,  as  suggested  in  the 
preceding  chapter,  and  to  have  each  map  mounted  on  heavy  card- 
board after  it  has  been  colored.  By  means  of  this  mounting  the 
maps  are  prevented  from  tearing,  and  can  be  kept  smooth  and  in 
good  condition. 

The  scheme  of  coloring  may  be  varied  to  suit  the  fancy  of  the 
scholars,  for  the  preparation  of  these  permanent  illustrations  may 
well  be  intrusted  to  those  of  the  class  who  are  especially  interested 
in  the  work,  and  who  are  skillful  in  their  use  of  the  paint  brush. 
The  work  is  really  very  simple  and  needs  only  ordinary  care.  As 
soon  as  the  drawing  of  isotherms  and  the  coloring  of  isothermal 
charts  have  been  sufficiently  studied,  the  teacher  should  hang  up 
the  daily  weather  map  each  day  (or  a  blank  map  with  the  isotherms 
for  the  day  drawn  on  it),  and  should  call  attention  to  the  tempera- 
ture changes  from  day  to  day.  In  this  way  the  facts  of  actual 
temperature  changes  experienced  by  the  class  will  be  associated 
with  the  larger  temperature  changes  shown  on  the  weather  maps. 

The  sections  on  temperature  gradients  may  be  postponed,  if  the 
teacher  deems  it  advisable,  until  other  matters  of  a  simpler  nature 
are  passed.  The  idea  of  rates  of  change  is  not  an  easy  one  for 
students  to  grasp,  and  it  is  far  better  to  postpone  the  consideration 
of  this  subject  than  to  involve  the  class  in  any  confusion  at  this 
stage.  It  is  a  mistake,  however,  to  omit  these  sections  altogether, 
as  a  clear  conception  of  the  principle  of  rates  is  a  valuable  part  of  a 
student's  mental  equipment.  It  is  a  good  plan,  in  the  exercises  on 


APPENDIX    A. 


181 


lines  of  temperature  decrease,  to  have  maps  prepared  with  faint 
isotherms  and  heavy  lines  of  temperature  decrease,  in  order  to 
emphasize  the  idea  of  change  of  temperature,  in  contrast  to  the 
idea  of  constancy  of  temperature  expressed  by  the  isotherms. 


CHAPTER   VI. 

In  the  work  on  the  wind  charts  it  is  essential  to  proceed  very 
slowly,  in  order  that  the  best  results  may  be  obtained  by  the  pupils. 
Some  of  the  aberrant  wind  courses,  which  complicate  the  discovery 
of  the  cyclonic  and  anticyclonic  spirals,  may  well  be  omitted  in  the 
case  of  the  younger  classes,  and  considerable  assistance  in  facilitat- 
ing these  discoveries  may  be  given  by  suggestions  as  to  adding  inter- 
mediate dotted  wind  arrows,  in  sympathy  with  the  observed  wind 
directions.  The  anticyclonic  systems  are  always  much  more  diffi- 
cult to  discover  than  the  cyclonic,  and  care  should  be  taken  to  assist 
the  class  in  this  matter  as  much  as  seems  necessary. 

The  questions  in  the  text  are  merely  suggestive,  and  are  by  no 
means  as  numerous  as  it  would  be  well  to  make  them  in  the  class. 
The  discovery  of  the  spirals  will  probably  be  made  by  degrees. 
The  concise  formulation  of  the  facts  discovered  will  furnish  excel- 
lent basis  for  exercises  in  writing. 

It  is  interesting  to  note  that  the  discussion  as  to  whether  the 
winds  blow  circularly  around,  or  radially  in  towards,  centers  of  low 
pressure,  which  usually  comes  up  in  every  class  in  meteorology  at 
some  stage  in  the  study,  was  carried  on  in  a  very  animated  way  about 
the  middle  of  the  present  century  in  this  country.  Two  noted 
American  meteorologists,  Redfield  and  Espy,  and  their  respective 
followers,  took  opposite  sides  in  this  controversy,  Redfield  maintain- 
ing at  the  start  that  the  winds  moved  in  circles,  and  Espy  main- 
taining that  they  followed  radial  courses.  The  truth  lay  between 
the  two. 

CHAPTER   VII. 

The  study  of  atmospheric  pressure  is  not  easy,  because  the  pupils 
cannot  perceive  the  changes  in  pressure  from  day  to  day  by  their 


182  APPENDIX    A. 

unaided  senses.  Especially  difficult  does  this  study  become  if  the 
class  has  not  already  had  some  practice  in  making  barometer  read- 
ings. When  this  observational  work  has  not  preceded  the  con- 
sideration of  the  isobaric  lines  of  the  daily  weather  maps,  the 
teacher  should  introduce  the  subject  of  pressure  very  carefully. 
The  experiment  with  the  Torricellian  tube  will  show  the  class  some- 
thing of  the  reality  of  atmospheric  pressure,  and  the  variations  in 
this  pressure  from  day  to  day  can  readily  be  made  apparent  by 
means  of  a  few  barometer  readings,  if  time  cannot  be  spared  for  a 
regular  and  continued  series  of  barometer  observations.  A  word 
may  be  said  as  to  the  correction  of  barometric  readings  for  local 
influences,  in  order  to  make  these  readings  comparable,  if  this 
matter  has  not  been  previously  met  with,  but  care  should  be  taken 
not  to  confuse  the  younger  pupils  too  much  with  explanations  at 
this  stage  of  the  work.  It  may  be  well  to  omit  this  point  unless 
it  is  brought  up  by  some  pupil.  The  questions  asked  in  the  text 
are  merely  suggestive.  They  may  be  added  to  and  varied  at  the 
discretion  of  the  teacher. 

Lines  of  pressure-decrease  should  be  drawn  on  all  isobaric  charts 
studied  in  the  class,  as  they  are  highly  instructive.  When  the 
isobars  are  near  together,  these  lines  of  pressure-decrease  may  be 
drawn  heavier,  to  indicate  a  steeper  gradient.  The  convergence  of 
these  lines  towards  regions  of  low  pressure,  and  their  divergence 
from  regions  of  high  pressure,  seen  on  every  map  on  which  these 
gradients  are  drawn,  emphasizes  an  important  lesson.  Before  meas- 
uring rates  of  pressure-decrease  by  means  of  a  scale,  considerable 
practice  should  be  given  in  the  study,  by  means  of  the  eye,  of  the 
rapidity  or  slowness  of  decrease  of  pressure,  as  shown  by  the  heavier 
or  lighter  lines  of  pressure-decrease.  When  the  broad  facts  of  dif- 
fering rates  are  comprehended,  then  the  actual  measurement  of 
these  rates  is  a  comparatively  easy  matter.  In  any  case,  however, 
an  appreciation  of  these  rates  of  change  is  not  always  readily  gained, 
even  by  older  scholars  in  the  high  school.  It  is,  therefore,  of  prime 
importance  to  proceed  very  slowly  indeed  at  this  point,  and  to  have 
every  step  fully  understood  before  another  step  is  taken. 

The  instructions  in  the  text  for  measuring  rates  of  pressure-decrease 
are,  that  these  rates  shall  be  recorded  as  so  many  hundredths  of  an 


APPENDIX    A.  183 

inch  of  change  of  pressure  in  one  latitude-degree.  This  is  done 
for  the  sake  of  simplicity.  If  this  rate  is  expressed  in  hundredths 
of  an  inch  of  pressure  in  a  quarter  of  a  latitude-degree  of  distance, 
the  numerical  value  is  the  same  as  if  expressed  in  millimeters  of 
pressure  per  latitude-degree  of  distance. 


CHAPTER    VIII. 

The  fact  of  the  prevalence  of  different  kinds  of  weather  over  the 
country  at  the  same  time  is  of  great  importance.  It  should  be 
strongly  emphasized  by  the  teacher  in  the  course  of  the  discussion 
of  the  maps  of  weather  distribution.  Additional  exercises  of  the 
same  sort  may  be  given  to  advantage,  by  letting  the  class  plot  and 
study  the  weather  signs  taken  from  any  current  weather  map.  In- 
structive lessons  may  be  taught  by  talking  over,  in  the  class,  the 
different  ways  in  which  people  all  over  the  country  are  affected  by 
the  character  of  the  weather  that  happens  to  prevail  where  they 
are. 

CHAPTERS    IX-XVIII. 

The  correlation  exercises  will,  as  a  whole,  teach  few  entirely  new 
facts  to  the  brighter  scholars  who  have  faithfully  completed  the 
preceding  work  in  observations  and  in  the  construction  and  study 
of  the  daily  weather  maps.  These  exercises  do,  however,  lead  to 
detailed  examination  and  to  the  careful  working  out  of  the  relations 
which  may  have  been  previously  noticed  in  a  general  way  only. 
They  give  the  repeated  illustration  which  is  necessary  in  order  to 
impress  firmly  on  the  mind  the  lesson  that  the  weather  map  has  to 
teach. 

It  is  a  good  plan  to  let  different  scholars  work  out  the  problems 
for  different  months.  The  results  reached  in  each  case  should  be 
discussed  in  the  class,  and  thus  each  member  may  have  the  double 
advantage  of  working  out  his  own  problem,  and  of  profiting  by  the 
work  done  by  his  fellows.  Throughout  these  exercises  care  should 
be  taken  to  have  weather  maps  of  all  months  studied.  The  exercise 
on  the  correlation  of  the  velocity  of  the  wind  with  the  pressure 


184  APPENDIX    A. 

cannot  be  undertaken  unless  the  work  on  temperature  and  pressure 
gradients  (Chapters  V  and  VII)  has  been  completed. 


CHAPTERS   XX-XXV. 

It  is  not  expected  that  any  one  scholar  can  accomplish  all  that  is 
here  outlined.  Examples  may  be  selected  from  the  list,  as  oppor- 
tunity offers,  so  that  each  scholar  shall  become  familiar  with  several 
problems. 

Few  of  the  problems  suggested  call  for  continuous  routine  obser- 
vation at  fixed  hours.  They  require,  on  the  other  hand,  an  intel- 
ligent examination  of  ordinary  weather  phenomena,  with  special 
reference  to  discovering  their  explanation.  In  most  of  the  prob- 
lems a  small  number  of  observations  will  suffice.  Und,er  the 
supervision  of  the  teacher,  different  problems  may  be  assigned  to 
the  several  members  of  a  class ;  or  several  scholars  may  work  on 
different  parts  of  the  same  problem,  exchanging  records  in  order 
to  save  time.  All  the  scholars  should  have  a  general  knowledge  of 
the  results  which  have  been  obtained  from  the  observations 
made  by  the  other  members  of  their  class.  The  teacher  will 
use  his  discretion  in  arranging  the  order  of  the  problems,  and 
in  selecting  those  that  are  best  suited  to  the  season  in  which  the 
work  is  done,  to  the  locality  in  which  the  school  is  situated. 
and  to  the  facilities  and  apparatus  at  command.  Although  the 
variety  of  accessible  problems  is  less  in  city  schools  than  in 
country  schools,  much  may  be  done  in  the  city  as  well  as  in  the 
country. 

The  opportunities  for  carrying  out  such  observational  work  vary 
so  much  in  different  schools  that  it  is  impossible  to  give  specific 
instructions,  which  shall  be  available  in  all  cases.  Some  general 
suggestions  are  therefore  given,  which  the  teacher  may  supplement 
by  more  detailed  instructions  framed  to  fit  the  particular  circum- 
stances of  each  case. 

A  review  of  the  headings  of  the  different  problems  shows  that  a 
very  general  correlation  exists  among  them,  whereby  the  subjects 
of  every  heading  are  associated  with  those  of  nearly  every  other. 


APPENDIX    A.  185 

Iii  other  words,  every  weather  element  is  treated  as  a  function  of 
several  other  elements.  It  follows  from  this  that  the  variety 
of  work  here  outlined  is  more  apparent  than  real,  and  that 
man}"  problems  which  appear  from  their  wording  to  be  entirely 
new  are  in  large  part  rearrangements  of  problems  previously 
encountered. 


APPENDIX    B. 


THE  EQUIPMENT   OF   A   METEOROLOGICAL   LABORATORY. 

A.     INSTRUMENTS. 

Exposed  Thermometer  (United  States  Weather  Bureau  pattern), 
with  brass  support,  $2.75. 

Maximum  and  Minimum  Thermometers  (United  States  Weather 
Bureau  pattern),  mounted  together  on  one  board,  $6.25. 

Wet  and  Dry  Bulb  Thermometers  (United  States  Weather  Bureau 
pattern),  mounted  on  one  board,  complete  with  water  cup,  $6.50. 

Sling  Psychrometer  (designed  by  Professor  C.  F.  Marvin,  of 
the  United  States  Weather  Bureau),  consisting  of  two  exposed 
mercurial  thermometers,  mounted  on  an  aluminum  back,  and  pro- 
vided with  polished,  turned  hard-wood  handle  and  brass  trim- 
mings, $5.00. 

Sling  Psychrometer,  consisting  of  two  cylindrical  bulb  thermom- 
eters, mounted  one  a  little  above  the  other  upon  a  light  brass  frame, 
with  a  perforated  guard  to  protect  the  bulbs  while  swinging,  but 
which  can  be  raised  (by  sliding  upon  the  frame)  for  the  purpose  of 
moistening  the  linen  covering  of  the  wet  bulb.  Much  less  liable  to 
be  broken  than  the  Weather  Bureau  pattern.  $5.00. 

Rain  Gauge  (United  States  Weather  Bureau  standard),  8  inches 
in  diameter,  complete,  with  measuring  stick,  $5.25. 

Rain  Gauge,  3  inches  in  diameter,  with  overflow  and  measuring 
stick,  $1.25. 

Wind  Vane  (United  States  Weather  Bureau  pattern),  $10.00. 

Anemometer  (United  States  Weather  Bureau  pattern),  with  indi- 
cator, aluminum  cups,  and  electrical  attachment,  $25.00. 

The  same,  with  painted  brass  cups,  $23.00. 

186 


APPENDIX    B.  187 

Anemometer  Register  (United  States  Weather  Bureau  pattern), 
with  pen  and  ink  attachment,  $35.00. 

The  same,  with  pencil  attachment  (old  style),  $24.00. 

Aneroid  Barometer  (for  meteorological  work),  $14.00 -$16.00. 

NOTE.  —  Much  cheaper  aneroids  can  be  purchased,  and  may  be  used  to  some 
advantage  in  the  simpler  observations  in  schools. 

Mercurial  Barometer  (Standard  United  States  Weather  Bureau 
pattern),  complete  with  attached  thermometer,  vernier,  etc.,  $30.00  - 
$33.00. 

NOTE.  —  The  above  instruments,  as  used  by  the  United  States  Weather 
Bureau,  are  made  by  H.  J.  Green,  1191  Bedford  Avenue,  Brooklyn,  N.  Y. 
The  prices  are  those  given  in  Green's  latest  catalogue. 

Mercurial  Barometer.  New  improved  form,  especially  designed 
for  school  use.  Mounted  on  mahogany  back.  Scale  engraved  on 
aluminum.  Divisions  of  scale  on  metric  and  English  systems. 
No  vernier,  $5.75. 

(L.  E.  Knott  Apparatus  Co.,  14  Ashburton  Place,  Boston,  Mass.) 

Thermograph  (designed  by  Dr.  Daniel  Draper,  of  New  York). 
Consists  of  a  bimetallic  thermometer  in  a  case  which  carries  a  disk, 
with  a  chart  upon  its  axle  instead  of  hands  like  the  ordinary  clock. 
A  pen  (resting  on  the  face  of  the  disk)  registers  the  fluctuations  of 
temperature  as  the  chart  is  carried  around.  Sizes,  14  x  20  inches, 
$30.00 ;  10  x  14  inches,  $15.00.  This  instrument  may  be  purchased 
of  H.  J.  Green. 

Thermograph.  Self-recording  thermometer  (as  adopted  by  the 
United  States  Weather  Bureau),  made  by  Richard  Freres,  of  Paris. 
Eecords  continuously  on  a  sheet  of  paper  wound  around  a  revolving 
drum,  which  is  driven  by  clock-work  inside.  Standard  size  (with- 
out duty),  $30.00. 

Barograph.  Self-recording  barometer  (as  adopted  by  the  United 
States  Weather  Bureau),  made  by  Eichard  Freres,  of  Paris.  Similar 
in  general  arrangement  to  the  thermograph.  Standard  size  (with- 
out duty),  $27.60. 

These  last  two  instruments  can  be  procured  through  Glaenzer 
Freres  &  Eheinboldt,  26  &  28  Washington  Place,  New  York  City. 

Instrument  Shelter  (standard  United  States  Weather  Bureau  pat- 
tern) will  hold  a  set  of  maximum  and  minimum  thermometers, 


188  APPENDIX    B. 

psychrometer,  and  a  thermograph.  May  be  set  up  on  top  of  posts 
driven  into  the  ground,  or  may  be  attached  to  a  wall,  $18.00. 

Barometer  Box,  for  the  standard  mercurial  barometer.  Made  of 
mahogany,  with  glass  panels  on  front  and  sides ;  lock  and  key,  and 
with  fittings  complete,  $8.50. 

These  may  be  purchased  of  H.  J.  Green. 

B.     TEXT-BOOKS. 

The  Story  of  the  Earth's  Atmosphere.  DOUGLAS  ARCHIBALD. 
New  York,  D.  Appleton  &  Co.,  1898.  18mo,  pp.  194.  40  cents. 

To  be  recommended  to  the  general  reader  who  wishes  to  gain  some 
knowledge  of  meteorology  quickly.  Not  a  text-book.  Contains  a 
chapter  on  "  Flight  in  the  Atmosphere." 

Elementary  Meteorology.  WILLIAM  MORRIS  DAVIS.  Boston, 
Ginn  &  Co.,  1898.  8vo,  pp.  355.  $2.50. 

The  most  complete  of  the  modern  text-books,  and  the  best  adapted 
for  use  in  the  systematic  teaching  of  meteorology.  The  modern 
views  are  presented  clearly  and  without  the  use  of  mathematics. 
Portions  of  it  are  somewhat  too  advanced  for  school  study,  but 
teachers  will  find  it  invaluable  as  a  reference  book  in  directing 
the  laboratory  work,  and  in  answering  the  questions  of  school 
classes. 

A  Popular  Treatise  on  the  Winds.  WILLIAM  FERREL.  New 
York,  John  Wiley  &  Sons,  1890.  8vo,  pp.  505.  $3.40. 

This  can  hardly  be  regarded  as  a  popular  treatise.  It  embodies, 
in  condensed  and  chiefly  non-mathematical  form,  the  results  of 
FerreP  s  researches  during  his  long  and  profound  study  of  the 
general  circulation  and  phenomena  of  the  atmosphere.  Teachers 
who  advance  far  into  meteorology  will  find  this  book  indispensable. 
It  is  not  at  all  suited  for  general  class-room  use. 

American  Weather.  A.  W.  GREELY.  New  York,  Dodd,  Mead 
&  Co.,  1888.  8vo,  pp.  286.  Out  of  print,  but  secondhand  copies 
are  probably  obtainable. 

Deals,  as  the  title  implies,  especially  with  the  weather  phenomena 
of  the  United  States.  Contains  brief  accounts  of  individual  hot  and 
cold  waves,  hurricanes,  blizzards  and  tornadoes,  and  gives  specific 


APPENDIX    B. 

data  concerning  maxima  and  minima  of  temperature,  precipitation, 
etc.,  in  the  United  States. 

Meteorology:  Practical  and  Applied.  JOHN  WILLIAM  MOOKK. 
London,  F.  J.  Rebman,  1894.  8vo,  pp.  445.  8  shillings. 

A  readable  book.  Considerable  space  is  given  to  instrumental 
meteorology.  Contains  chapters  on  the  climate  of  the  British  Isles 
and  on  the  relations  of  weather  and  disease  in  the  British  Isles. 
Especially  adapted  for  the  use  of  English  readers. 

Elementary  Meteorology.  ROBERT  H.  SCOTT.  International  Scien- 
tific Series.  London,  Kegan  Paul,  Trench  &  Co.,  1885;  Boston, 
A.  A.  W7aterman  &  Co.,  1889.  8vo,  pp.  410.  6  shillings. 

The  standard  text-book  in  Great  Britain.  The  author  is  secretary 
to  the  Meteorological  Council  of  the  Royal  Society.  Fairly  com- 
plete, but  now  somewhat  out  of  date  in  some  portions.  It  is  a 
useful  book  in  a  meteorological  library,  but  does  not  treat  the  sub- 
ject in  a  way  very  helpful  to  the  teacher. 

Meteorology.  THOMAS  RUSSELL.  New  York,  The  Macmillan 
Company,  1895.  8vo,  pp.  277. 

Brief  and  incomplete  as  a  text-book  of  meteorology,  but  contain- 
ing a  very  comprehensive  account,  fully  illustrated,  of  rivers  and 
floods  in  the  United  States,  and  their  prediction. 

Elementary  Meteorology.  FRANK  WALDO.  New  York,  Amer- 
ican Book  Company,  1896.  8vo,  pp.  373.  90  cents. 

A  compact  summary.    Useful  to  teachers  as  a  handy  reference  book . 

Modern  Meteorology.  FRANK  WALDO.  New  York,  Charles 
Scribner's  Sons,  1893.  8vo,  pp.  460.  $1.25. 

Very  complete  account  of  meteorological  apparatus  and  methods, 
and  admirable  summary  of  recent  German  studies  of  the  thermo- 
dynamics and  general  motions  of  the  atmosphere. 

C.     INSTRUCTIONS  IN  THE  USE  OF  INSTRUMENTS. 

Instructions  for  Voluntary  Observers.  1899.  8vo,  pp.  23. 
Brief  instructions  for  taking  and  recording  observations  of  tem- 
perature and  precipitation  with  ordinary  and  maximum  and  mini- 
mum thermometers  and  with  the  rain  gauge. 


190  APPENDIX    B. 

Barometers  and  the  Measurement  of  Atmospheric  Pressure. 
C.  F.  MARVIN.  1894.  8vo,  pp.  74.  A  pamphlet  of  information 
respecting  the  theory  and  construction  of  barometers  in  general, 
with  summary  of  instructions  for  the  care  and.  use  of  the  standard 
Weather  Bureau  instruments. 

Instructions  for  Obtaining  and  Tabulating  Records  from  Recording 
Instruments.  1898.  8vo,  pp.  31.  Contains  directions  concerning 
the  care  and  use  of  the  Richard  thermograph  and  barograph. 

NOTE.  — These  pamphlets  are  prepared  under  the  direction  of  Professor 
Willis  L.  Moore,  Chief  of  the  United  States  Weather  Bureau,  and  are  pub- 
lished, under  authority  of  the  Secretary  of  Agriculture,  by  the  Weather 
Bureau.  They  will  be  found  the  best  guides  in  making  observations,  the 
care  of  instruments,  etc. 

I).     JOURNALS,  ETC. 

Monthly  Weather  Revieiv.  Prepared  under  the  direction  of 
Willis  L.  Moore,  Chief  of  Weather  Bureau,  Professor  Cleveland 
Abbe,  Editor.  United  States  Department  of  Agriculture,  Weather 
Bureau,  Washington,  D.  C.  10  cents  a  copy. 

An  invaluable  publication  for  teachers  and  students  alike.  Con- 
tains complete  meteorological  summaries  for  each  month ;  accounts 
of  all  notable  storms,  cold  and  hot  waves,  etc. ;  and  a  large  number 
of  articles  on  a  wide  range  of  meteorological  subjects.  The  charts 
show  the  tracks  of  areas  of  high  and  low  pressure  which  crossed 
the  United  States  during  the  month,  the  total  precipitation,  sea- 
level  pressure,  temperature  and  surface  winds,  percentage  of  sun- 
shine, etc.,  for  the  month.  Other  charts  are  also  frequently  added. 

The  Journal  of  School  Geography.  Professor  Richard  E.  Dodge, 
Teachers  College,  Columbia  University,  New  York  City,  Editor. 
Publication  Office,  41  No.  Queen  Street,  Lancaster,  Pa.  Ten  numbers 
a  year.  $1.00  per  annum. 

A  monthly  journal  devoted  to  the  interests  of  the  common  school 
teacher  of  geography.  Contains  numerous  articles  and  notes  on 
meteorological  and  climatological  subjects. 

Science.  Edited  by  Professor  J.  McK.  Cattell,  Columbia  Univer- 
sity. New  York  City.  New  York,  The  Macmillan  Company. 
Weekly.  $5.00  per  annum. 


APPENDIX    B. 

Devoted  to  the  advancement  of  all  sciences.  Contains  brief  Cur- 
rent Notes  on  Meteorology,  which  summarize  the  more  important 
meteorological  publications. 

Monthly  Bulletins  of  the  Climate  and  Crop  Service  of  the  Weather 
Bureau. 

These  Bulletins  are  issued  every  month  at  the  central  office  of  the 
Weather  Bureau  in  each  State,  under  the  direction  of  the  Section 
Director  of  the  Climate  and  Crop  Service  in  that  State.  They  con- 
tain meteorological  data  for  the  month,  and  frequently  notes  of 
interest.  The  annual  summaries  are  especially  valuable. 

E.     CHAIITS. 

Daily  Weather  Maps.  These  are  published  at  the  central  office 
of  the  Weather  Bureau  in  Washington,  and  at  eighty-four  other 
stations  of  the  Bureau  throughout  the  United  States.  It  is  best  to 
have  the  daily  maps  sent  from  the  nearest  map-publishing  station, 
and  not  from  Washington,  as  the  delay  in  the  latter  case  is  often 
so  great  that  much  of  the  immediate  value  of  the  maps  is  lost. 

Climate  and  Crop  Bulletin  of  the  United  States  Weather  Bureau. 
Washington,  D.  C.  Monthly. 

Chart  showing,  by  means  of  small  maps,  the  actual  precipitation, 
departures  from  normal  precipitation,  departures  from  normal  tem- 
perature, and  maximum  and  minimum  temperatures.  Also  a  printed 
summary  of  the  weather  and  of  the  crop  conditions  in  the  different 
sections  of  the  United  States.  Issued  on  the  first  of  each  month. 

Snow  and  Ice  Chart  of  the  United  States  Weather  Bureau. 
Washington,  D.  C.  Weekly  during  the  winter  season. 

Based  on  data  from  regular  Weather  Bureau  stations,  supple- 
mented by  reports  from  selected  voluntary  observers.  Shows,  by 
shading,  the  area  covered  with  snow  at  8  P.M.  each  Tuesday  during 
the  winter,  and  by  lines,  the  depth  of  snow  in  inches.  Explanatory 
tables  and  text  accompany  the  chart. 

Storm  Bulletin  of  the  United  States  Weather  Bureau.  Wash- 
ington, D.  C.  Issued  at  irregular  intervals. 

Charts,  with  text,  illustrating  the  history  of  individual  notable 
storms. 


192  APPENDIX    B. 

Pilot  Chart  of  the  North  Atlantic  and  North  Pacific  Oceans. 
Hydrographic  Office,  Bureau  of  Equipment,  Department  of  the 
Navy,  Washington,  D.  C.  Monthly.  Price  10  cents  a  copy. 

Shows  calms  and  prevailing  winds,  ocean  currents,  regions  of  fog 
and  equatorial  rains,  the  positions  of  icebergs  and  wrecks,  steamship 
and  sailing  routes,  storm  tracks,  magnetic  variation,  etc.  Also  gives 
isobars  and  isotherms  and  a  forecast  for  the  month  succeeding  the  date 
of  publication,  and  a  review  of  the  weather  over  the  oceans  for  the 
preceding  month.  Supplementary  charts  are  occasionally  issued. 

Rainfall  and  Snow  of  the  United  States  as  compiled  to  the  End 
of  1891,  with  Annual,  Seasonal,  Monthly,  and  other  Charts.  MARK 
W.  HARRINGTON.  United  States  Department  of  Agriculture, 
Weather  Bureau,  Bulletin  C,  Washington,  D.  0.  1894.  Atlas, 
18x24  inches.  Charts  23.  Text,  4-80  pp. 

Contains  twenty -three  charts  as  follows :  Monthly  rainfall,  sea- 
sonal rainfall,  annual  rainfall,  monthly  snowfall,  monthly  maxima 
of  rainfall,  rainy  seasons,  details  of  rainfall,  details  of  occurrence  of 
thunderstorms.  Well  adapted  to  serve  as  illustrations  for  use  in 
the  class-room.  The  text  is  explanatory,  and  is  published  separately 
in  quarto  form. 

Rainfall  of  the  United  States,  ivith  Annual,  Seasonal,  and  other 
Charts.  ALFRED  J.  HENRY.  United  States  Department  of  Agri- 
culture, Weather  Bureau,  Bulletin  D,  Washington,  D.  C.  1897. 
9±x  11£  inches.  Pp.  58.  Charts  10.  Plates  III. 

A  more  recent  publication  than  the  preceding  one,  the  averages 
having  been  compiled  to  the  end  of  1896.  The  charts  are  smaller 
than  most  of  those  in  Bulletin  C,  and  therefore  not  so  well  adapted 
for  class-room  illustration.  The  chart  of  mean  annual  precipitation 
is  the  latest  and  best  published.  The  rainfall  of  the  crop-growing 
season  receives  separate  treatment,  and  is  illustrated  by  means  of 
two  charts.  The  discussion  in  the  text  is  excellent. 

F.     METEOROLOGICAL  TABLES. 

Smithsonian  Meteorological   Tables.     Smithsonian  Miscellaneous 
Collections,  844.     Washington,  D.  C.     1893.     8vo.     Pp.  262. 
A  very  complete  set  of  tables. 


APPENDIX    B.  193 

Handbook  of  Meteorological  Tables.  H.  A.  HAZEN  (of  the  United 
States  Weather  Bureau).  Washington,  D.  C.  1888.  8vo.  Pp.  127. 
$1.50. 

Contains  forty -seven  tables,  comprising  all  that  are  needed  by  the 
working  meteorologist.  Includes  tables  for  Fahrenheit  and  Centi- 
grade conversions,  for  barometric  hypsometry  and  reduction  to  sea 
level,  for  the  psychrometer,  etc. 

Tables  for  Obtaining  the  Temperature  of  the  Dew-Point,  Relative 
Humidity,  etc.  United  States  Department  of  Agriculture,  Weather 
Bureau,  Washington,  D.  C.  1897.  8vo.  Pp.  29. 

These  are  the  tables  now  in  use  by  the  Weather  Bureau. 

G.     ILLUSTRATIONS. 

Classification  of  Clouds  for  the  Weather  Observers  of  the  Hydro- 
graphic  Office.  Hydrographic  Office,  Bureau  of  Navigation,  Depart- 
ment of  the  Navy,  Washington,  D.  C.  1897.  Sheet  of  twelve 
colored  views.  Price  40  cents.  In  book  form,  with  descriptive  text, 
$1.00. 

An  excellent  set  of  cloud  views,  classified  according  to  the  Inter- 
national Nomenclature.  The  text  describes  the  various  cloud  forms 
and  shows  their  value  as  weather  prognostics.  An  attractive  addi- 
tion to  the  furnishings  of  a  school-room. 

Selected  List  of  Cloud  Photographs  and  Lantern  Slides. 

Consists  of  twenty -eight  photographs,  and  the  same  number  of 
lantern  slides,  of  the  typical  cloud  forms,  selected  by  the  present 
writer  from  the  collection  in  the  Physical  Geography  Laboratory  of 
Harvard  University.  .The  photographs  (20  cents  each,  mounted) 
and  slides  (40  cents  each)  may  be  purchased  of  E.  E.  Howell,  612 
17th  Street,  N.  W.,  Washington,  D.  C.  A  description  of  these  views 
was  published  in  the  American  Meteorological  Journal  for  July, 
1894  (Boston,  Mass.,  Ginn  &  Company). 

Photographs.  Photographs  of  miscellaneous  meteorological  phe- 
nomena, such  as  snow  and  ice  storms,  damage  by  storm-waves  or 
high  winds,  wind-blown  trees,  lightning,  etc.,  may  often  be  pur- 
chased of  local  dealers.  They  add  to  the  attractiveness  of  a  school- 
room and  furnish  excellent  illustrations  in  teaching. 


194  APPENDIX    B. 

H.     GENERAL. 

The  following  Bulletins  of  the  Weather  Bureau  may  be  found 
useful  as  reference  books : 

No.  1.  Notes  on  the  Climate  and  Meteorology  of  Death  Valley, 
California.  MARK  W.  HARRINGTON.  8vo.  1892.  Pp.  50. 

No.  8.  Report  on  the  Climatology  of  the  Cotton  Plant.  P.  H. 
MELL.  8vo.  1893.  Pp.  68. 

No.  10.  The  Climate  of  Chicago.  H.  A.  HAZEN.  8vo.  1893. 
Pp.  137. 

No.  11.  Report  of  the  International  Meteorological  Congress  held 
at  Chicago,  III.,  Aug.  21-24,  1893.  8vo.  Pt.  I,  1894,  pp.  206. 
Pt.  II,  1895,  pp.  583.  Pt.  Ill,  1896,  pp.  772.  Pt.  IV,  not  yet 
issued. 

No.  15.  Protection  from  Lightning.  ALEXANDER  Me  AD  IE.  8vo. 
1895.  Pp.26. 

No.  17.  The  Work  of  the  Weather  Bureau  in  Connection  with 
the  Rivers  of  the  United  States.  WILLIS  L.  MOORE.  8vo.  1896. 
Pp.  106. 

No.  19.  Report  on  the  Relative  Humidity  of  Southern  New  Eng- 
land and  Other  Localities.  A.  J.  HENRY.  8vo.  1896.  Pp.  23. 

No.  20.  Storms,  Storm  Tracks  and  Weather  Forecasting.  FRANK 
H.  BIGELOW.  8vo.  1897.  Pp.87. 

No.  21.  Climate  of  Cuba.  Also,  A  Note  on  the  Wreather  of  Manila . 
W.  F.  R.  PHILLIPS.  8vo.  1898.  Pp.  23. 

No.  23.  Frost :  When  to  expect  it  and  how  to  lessen  the  Injury 
therefrom.  W.  H.  HAMMON.  8vo.  1899.  Pp.  37. 

No.  25.  Weather  Forecasting :  Some  Factp  Historical,  Practical, 
and  Theoretical.  WILLIS  L.  MOORE.  8vo.  1899.  Pp.  16. 

No.  26.  Lightning  and  the  Electricity  of  the  Air.  In  two  parts. 
A.  G.  McAoiE  and  A.  J.  HENRY.  8vo.  1899.  Pp.  74. 

The  following  miscellaneous  publications  of  the  Weather  Bureau 
may  also  prove  of  value. 

Injury  from  Frost  and  Methods  of  Protection.  W.  H.  HAMMON. 
8vo.  1896.  Pp.  12. 

Some  Climatic  Features  of  the  Arid  Regions.  WILLIS  L.  MOORE. 
8vo.  1896.  Pp.  19. 


APPENDIX    B.  195 

Investigation  of  the  Cyclonic  Circulation  and  the  Translator*/ 
Movement  of  the  West  Indian  Hurricanes.  The  late  REV.  BENITO 
VINES,  S.  J.  8vo.  1898.  Pp.  34. 

Requests  for  weather  maps,  Bulletins,  and  other  publications  of 
the  Weather  Bureau  should  be  sent  to  the  Chief  of  the  Weather 
Bureau,  Washington,  D.  C.  All  requests  are  .dealt  with  on  their 
merits,  and  in  cases  where  it  is  deemed  that  effective  use  will  be 
made  of  the  publications  they  are  usually  sent  free  of  charge. 


Anemometer,  38-41. 
Aneroid  barometer,  23,  24. 
Anticyclones,  76. 

—  and  pressure  changes,  141. 

—  and  temperature,  104-106. 

—  and  weather,  109-112. 

—  and  wind  circulation,  98-100. 

—  form  and  dimensions  of,  96-98. 

—  progression  of,  111,  112. 

—  tracks  of,  111,  112. 

B. 

Backing  winds,  118. 
Barograph,  36,  37. 

—  records,  37,  38. 
Barometer,  aneroid,  23,  24. 

—  corrections,  33,  34. 

—  mercurial,  19-23,  32,  33. 

reduction   to  freezing,  143,  144, 

166,  167. 
Barometer  reduction  to  sea  level,  144, 

145,  168-170. 
Buran,  75,  76. 
Buys-Ballot's  Law,  99. 

C. 

Cherrapunjee,  rainfall  at,  138. 
Clouds  and  upper  air  currents,  136, 

137. 
Clouds  as  weather  prognostics,  137. 

—  forms  and  changes  of,  136. 

—  movements  of,  41-43,  46,  95,  136, 
137. 

Clouds,  observations  of,  5,   9,  41-43, 

45,  46. 
Cold  wave,  62,  75,  76,  103,  106. 


Cold-wave  forecasts,  63. 

Correlations  of  weather  elements,  91- 

113. 
Cyclones,  76. 

—  and  pressure  changes,  141. 

—  and  temperature,  104-106. 

—  and  weather,  109-111,  113,  114. 
and  wind  circulation,  98-100. 

—  form  and  dimensions  of,  96-98. 

—  progression  of,  111,  112. 
-  tracks  of,  111,  113. 

—  tropical,  106. 

—  velocity  of,  112,  113. 

D. 

Dew,  134,  135. 

—  point,  30. 

-  tables,  142,  143,  146-155. 
Diurnal  variation  of  temperature,  125- 

127. 
Diurnal   variation   of   wind  velocity, 

130. 
Doldrums,  29. 


Equipment  of  a  meteorological  labora- 
tory, 186. 
Evaporation,  29. 

F. 

Fahrenheit,  12,  13. 
Fen-el's  Law,  93. 
Forecasts,  cold-wave,  63. 

-  weather,  49,  114-124. 
Franklin,  Benjamin,  114,  115. 
Frost,  135. 
warnings,  135. 


197 


198 


INDEX. 


G. 

Galileo,  12,  19. 
Gradients,  pressure,  82-85. 

—  temperature,  64-70. 

—  vertical  temperature,  129. 


Humidity,  29,  132-134. 

diurnal  variation  of,  133. 

—  relative,  30,  32,  45,  133. 

—  relative,  and  wind  direction,  133. 
-  tables,  143,  156-165. 

Hurricane,  100. 

I. 

Inversions  of  temperature,  129. 
Isobaric  charts,  76-82,  85,  95,  96. 
Isobars,  77-82,  121. 
Isothermal  charts,  63,  68-70. 
Isotherms,  55-57,  121. 


Land  and  sea  breezes,  3,  131,  132. 
Looniis,  97,  113. 

M. 

Meteorological  tables,  146-170. 

Mistral,  75. 

Monsoons,  100. 

Mountain  and  valley  winds,  131. 

N. 

Nephoscope,  41-43. 

O. 

Observational  meteorology,   problems 

in,  125-141. 
Observations,  advanced  instrumental, 

26-46. 
Observations,  cloud,  5,  9,  41-43. 

elementary  instrumental,  11-26. 

Observations,  non-instrumental,  1-10. 


Observations,  rainfall,  9,  10,  26. 

—  state  of  sky,  5,  9. 

—  temperature,  1-3,  25. 

-  wind,  3-5,  8,  9,  25. 

P. 

Pampero,  76,  104. 

Pascal,  21. 

Precipitation,  7,  9,  10,  138,  139. 

Pressure  and  wind  direction,  91-93. 

and  wind  velocity,  93-96. 

atmospheric,  19-21. 

—  charts,  76-83. 

—  cyclonic  variation  of,  140,  141. 

—  decrease  with  altitude,  139,  140. 
diurnal  variation  of,  140,  141. 

—  gradient,  82-85. 

Prevailing  westerly  winds,  93,  95,  96, 

112,  113. 
Psychrometer,  28. 

—  sling,  31. 
Purga,  75. 

R. 

Rain,  see  Precipitation. 

-  heavy,  138,  139. 

—  gauge,  15-17. 
Kainfall  records,  17,  18,  20. 


Sensible  temperatures,  29,  30. 
Sirocco,  103,  104,  133,  134. 
Sling  psychrometer,  31. 
Smudges,  135. 
State  of  sky,  5,  9,  45. 
Suggestions  to  teachers,  171. 

T. 

Temperature  charts,  58-60. 

—  distribution,  61-63. 

—  diurnal  range,  125-127. 

—  forecasts,  116. 
—  gradient,  64-70. 


INDEX. 


199 


Temperature  gradient,  vertical.  129. 
inversions,  129. 

—  maximum  and  minimum,  45. 

—  mean,  43,  45. 

—  observations,  1-3,  8-10, 25, 43, 45. 

-  range,  45,  125-127. 

-  sensible,  29,  30. 

—  vertical  distribution  of,  128,  129. 
Thermograph,  34-36. 

—  records,  35,  36. 
Thermometer,  12. 

—  attached,  33. 

—  maximum  and  minimum,  26-28. 

—  shelter,  13,  14. 

-  wet  and  dry  bulb,  28,  30,  31. 
Torricelli,  19,  20. 

Trade  winds,  93. 

V. 

Veering  winds,  118. 

Vernier,  33. 

Vertical  temperature  gradient,  129. 

W. 

Water  vapor,  28. 
Weather,  85-90. 
and  wind  direction,  106-109. 

—  changes,  sequence  of,  113,  114. 
— _  charts,  85-90. 


Weather  forecasts,  49,  114-124. 

—  map  data,  90. 

—  maps,  47-51. 

—  prognostics,  137. 

—  signs,  85. 

—  temperate  zone,  88-90. 

—  torrid  zone,  88-90. 
Wind  charts,  70-75. 

—  direction  and  pressure,  91-93. 

—  and  relative  humidity,  133. 

—  and  temperature,  101-104. 

—  and  weather,  106-109. 
—  forecasts  of,  118. 

—  observations,  3-5,  25,  40,  41. 
rose,  103,  108,  109. 


-  velocity,  3,  40,  41,  45. 

—  and  pressure,  93-96. 
— .  diurnal  variation  of,  130. 

—  forecasts  of,  118. 

—  scale,  3. 

Winds  around  cyclones  and  anticy- 
clones, 98-100. 

Winds,  mountain  and  valley,  131, 
132. 

-  prevailing   westerly,    93,    95-96, 
112,  113. 

Winds,  trade,  93. 
Woeikof ,  69. 


14  DAY  USE 

RETURN  TO  DESK  FROM  WHICH  BORROWED 


EARTH  SCIENCES  LIBRARY 

This  book  is  due  on  the  last  date  stamped  below,  or 
on  the  date  to  which  renewed. 
Renewed  books  are  subject  to  immediate  recall. 

•ffSf 

General  Library 


